Bathroom bulb only giving off weak light
Fluoresent ballasts do go bad. If it is a very old fixture it may have a starter:a starter is usually a small tubular module with 2 pins on 1 end - a twist, usually CCW, and it will come out. Starters used to be very common; if you find one it is probably bad. If no starter, the ballast has failed (not uncommon).

Changing a ballast is a simple procedure: turn the power off, open the fixture and record the part number on the existing ballast. Find one online or locally. Usually there is a line diagram of the connections. If you get a different brand, you may need up to 6 wire nuts: again with the power off, cut the black and white at the ballast and connect to the new ballast. Next is the lamp connections - many mfr's use the same color-coded wiring. If the wires on the old ballast match the new one, finish cutting the old ballast out and wire nut the wires together. If the colors are different, compare the line drawings on the 2 ballasts and transfer to the new ballast using the diagram on the old ballast. This is what electricians do.

I have changed thousands of ballasts over the years and I still need to look at the instructions. Ballasts are a DIY item in my professional opinion.