Lights are on but motor won't run?

The brush on this model is controlled by the on/carpet/floor switch and will only work in the carpet position. If the suction motor is running then it could be the roller brush motor, but it's more likely to be the microswitch which stops the brush motor when the handle is returned to the upright position.

The Panasonic MC-5100 Series takes this belt here:

Panasonic MC-5127 Replacement Belts

You will have to access the motor by removing the entire bottom brush housing and tilting the vac motor shaft out of the housing. Once released you will have a main body and brush housing.
There are visible motor housing screws in the lower portion of the main housing and if I recall there may be twoscrews inside the bag chamber as well

Hi there. In Panasonic vacuums, there is a thermostat protection on the motor. It will automatically shut off the motor if there is no airflow through the motor. This is typically caused by a clogged hose. Typically, if your dust container is that full, there could be backup in the hose. Or the dust container being too full could also or otherwise be the problem. Check your hose for blockage, let the machine cool off, it should start up again

Motor overheated so I waited for the thermo switch to reset

disconnect the hose where it goes into the base and see if you have suction there.if you don't then your intake hose is clogged.thats the hose that your long stretch hose connects to.take a coat hanger and bend the tip so the end is rounded.work it down into the intake hose at the bottom and try to push the clog out through the bottom.make sure your filters are clean and the bag isn't full. please leave feedback

Is there a lever on the side that switches the unit between hard floors and carpet i.e. a switch that can turn the roller off? If so, check that the belt is sitting properly in that mechanism, and not rubbing on the side.

Other there may be a fault with the motor. Sounds like not an easy fix so may be worth the trip to see a Service Technician.

Panasonic MC-E 450UK upright vacuum cleaner.
Just had this problem myself...........easy to get at it when you know how.
Panasonic UK wouldn't help me so I spent some time working it out..........here it is.
Remove the hose from its mounting storage at the rear (called hose clip) where the main body of the vacuum cleaner meets the base of the handle assembly. Remove the screw at the rear of the handle assembly which is behind where the hose 'was' situated and is located inside the deep recess.
Using the soft part of your clenched hand as a hammer, knock the base of the handle assembly at the front and it will slide backwards off the main body of the vacuum cleaner. Don't be scared, give it a thump and keep moving it backwards with a few more knocks.
Next remove the two screws at the front which hold the mains cable and on/off switch cover in place.
Finally looking from the rear once more, lift the plastic 'lug' at the top of the cover and which is part of the cover. This will now allow you to release the cover and pull it forward.
There you go! Told you it was easy!
Watch out for the funny little spring at the front behind the cover in the tiny plastic tube where it connects to the small rubber tubing as it could possibly try to escape.
You now have complete access in order to cut the intermittent part of the cable out and remove it from the 'choc' block.
Happy fixing!!
Jack.

Sounds like a problem with the impeller. It is loose or broken. It is attached to the motor shaft.

Probably not the motor protector, its function is shut off the motor if it overheats. Most likely you have a clog or partial clog in the inlet down by the brush roll, or it is in the hose. But if the motor is running, and suction is low, with no clogs, then maybe the motor is running at a slower speed and will have to be replaced.