How to Troubleshoot a T12 Fixture?

make sure you turn your breaker off that supplies power to this light fixture.

most likely one of the wires in the tombstone came undone there should be 2 wires connected on each tombstone (thats what the white plastic is called that holds the bulb on the end) on a floresent lamp each bulb has 4 metall pins ( 2 on each end ) all 4 of those have a single wire going to the ballast. one side of the bulb will have 2 yellow wires and the other will have 2 blue wires, if it is a 2 bulb T-12 fixture it will also have 2 red wires, and another set of 2 yellow wires just make sure all those wires have a good connection in the tombstone.

it could also be a broken tombstone they cost about a dollar if that.

but i can only agree its easy to replace the ballast with a new T8 ballast.

this way you don`t have to replace the tombstones, if you switch t5 the bulb lenght is not the same you will have to replace the tombstone holder plate as well.

the best thing is just to replace the complete fixture and not have to worry about anything this is my recommendation anyways.

most those T12 have completed the life cycle and the T12 lamps will not be avail. much longer anyways its time to upgrade to a newer fixture. I would just go and buy a new T5 but make sure to buy a daylight color bulb not a 4100K lamp or even 7000K lamp. this is the color of the lamp you would want something that is good on your eyes not super cold blue and bright.