Where is the reset breaker located on the ml121rt? Lights have been tested and work with an alternate transformer but noting using terminals a & b on the ml121rt.

Something happened to the unit.
Good news is that lights are still ON.
Might be power surge. Or extra wire wrapped around transformer.
Clean up terminal with sandpaper.
If clock motor does not rotate, but lights are ON, then plug power pack into cheap wall timer.
Copy following link and read tips for getting power pack to last longer. One of these reasons may apply to problem.
Plus view warranty, and replacement trippers.
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Check circuit breaker.
Click fully off and then back on.
Check for burned smell at breaker or loose crackling sound that says breaker is bad.

Reset timer. Then Reprogram.
Possible geyser timer manual
http://waterheatertimer.org/pdf/Digital-Geyser-Timer 24H-7-Day-Timer-230V Brochure.pdf

This timer has a non-replaceable internal rechargeable battery.
If battery has gone bad, then timer is defunct.

Use manual override button to turn timer ON-OFF.
Then test across terminals 3 and 4 for 240Volts.
Turn timer OFF, and test across terminals 4 and 5 for 240Volts
If no voltage shows then timer does not have output.

If test shows 80% undervoltage, that can cause load not to activate, depending on type of load.
Have electrician check for voltage issues at power company transformer.

Unknown what load timer is connected to.
Max load for this timer at 240 volts is 3680 watts, 16 amps.
Motor load is 230 volt 8 amps.
Check label on load for watt or amp rating.
Volts x amps = watts
Watts divided by volts = amps

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Model number is not known.
I did not know that Kitchler transformers have a circuit breaker.
Usually they have an override switch.
Did you set the timer to override, what does your product manual say?
Is the transformer humming? Kitchler transformer are supposedly non-audible, but there should be some indicating sound. The humming sound indicates the transformer is working.
Does the unit have photoeye? Cover photoeye with black tape.
Test outlet with circuit analyzer.
Check outlet voltage: if you have 120Volt then output is 10% or 12Volts. If you have reduced voltage, then output is still 10% but might be lower than needed to operate bulbs.
Plug timer into different outlet.
Test output terminals with low voltage tester.
Did you have power outage or nearby lightning strike? Transformers are not protected from surge.
Suggest you contact kitchler.
Kitchler.com website is useless
888-412-4877
7711 E Pleasant Valley Rd
PO Box 318010
Cleveland, OH 44131
Kitchler products are probably warranted through the seller.
Kitchler product manuals are almost non-existent.
Scan your product operating manual and parts list into .pdf format and post on web for others to access.

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Try it... should work
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html#install
However minimum load for ML121 is 80% load rating.. so 80 watts is ok for 100 watt transformer... 60 watts is low and can cause early failure of transformer.
Gene

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There are two possible problems you could be facing (excluding the transformer going bad) If you have replaced light bulbs with higher wattage ones, you could be drawing too much current through the transformer. (It is common for MR-16 style lightbulbs to be 20w in landscape lights, but 50w for inside lights, and the local hardware store usually only carries the 50w replacement lamps) Each transformer has a specific amount of "watts" it can supply (anywhere from 100w to 1000w) If you exceed the supply wattage of the transformer, the breaker inside the transformer will trip. The quick and easy way to test this is to take a few of the lightbulbs out and see if the breaker will hold.
If you are positive that the total lamp wattage does not exceed the rating of the transformer the second possible problem (but less likely) is that you have a lamp socket or wire connection that is full of water. This water between the two conductors draws current just like a light bulb and can cause an overcurrent fault. This is usually more common if it starts to trip right after a heavy rain when the ground gets saturated with water.

Good Luck!

You connect the lights to the A and B terminal.

The C terminal is used for an optional photo control.

1) Disconnect the plug.
2) Remove the 3 hex screws holding the cover.
3) Make a note of all connections (taking a picture is even better).
4) Connect the black power wire to the red transformer wire.
5) Connect the white power wire to the white transformer wire.
6) Set your voltmeter to 20 or 50 volts AC (not DC and not less than 15 volts).
7) Plug the transformer in the wall.
8) Test the voltage coming from the A & B connectors at the back (polarity is irrelevant).
If you see 12 volts or thereabout, the problem is with the little switch (the one with the black and red wires that is activated by the trippers on the clock dial). I have the exact same problem but I have been unable to locate the part so I am going to try to clean the contact points and see if that works.

The PB912M10 timer clock motor is not sold separately. Aug 2010.
If power pack transformer still works: buy cheap plug-in timer, set power pack to override, and plug power pack into cheap timer.
If transformer no longer works, then it is not repairable.
With receipt, power pack might still be under warranty.
Open following link for power pack tips and resources:
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html

Turn switch on lamp to "ON"

Well, you're overloading the transformer by 30 watts. It's doing what it's supposed to do when overloaded...shut down. It's the _other_ (works fine) transformer that you should worry about possibly starting a fire, since it's not shutting down when overloaded.