If I understand correctly new dimmer is in different box than outdoor switch.
And outdoor switch controls different light than new dimmer.
Both new dimmer and outdoor light are on same circuit breaker, and that breaker is not tripped.
Outdoor problem could be related if loose wire inside dimmer box also feeds over to outdoor switch. More below.
Here's how I would troubleshoot problem.
1) Check outdoor bulb(s). This step can be big time saver.
2) Pull outdoor switch out of box a bit so you can see and test wires.
3) If wires are pushed into quick connects located on back of switch, these connects get loose. Moving switch usually causes loose wire to connect again, so turn switch ON-OFF and see if outdoor lights work. Sometimes switch works fine after quick connects reconnect. If quick connects are bad, use tiny screwdriver in slot next to connect-hole to release wire. And then connect wire to screw terminals on switch. Or replace switch.
4) Turn switch to OFF, use tester and test both screws on switch to bare ground wire. Tester should light up on one screw. If tester does not light up, then switch is not getting power, and that wire has to be tracked back to see where short is.
a) Look if one of the black switch wires is connected to other switch, or connected to other black wires inside same box. This wire might have come loose.
b) Look at white wire inside same box and make sure they are twisted together and covered with wire nut.
c) Now here is where dimmer could
be related ... if white wires are joined together in dimmer box, they could have come loose if they were not properly twisted together and covered with wire nut.http://waterheatertimer.org/How-to-twist-electric-wire.html
5) Turn switch to ON and test again. Tester should light up on both screws. If switch does not light up on both screws, then switch is bad. Replace switch.
Add a comment if you need more help.