There are many different types and sizes of fluorescent light tubes on the market. The most common for home use is the 2 pin variety, that comes in straight and looped (U shape) tubes.
When you notice that the light is not working, it is a good chance it’s the fluorescent tube, depending on the light, on some fixtures if one bulb is bad the other will not light because they are wired in a loop so they both need to be working.
Changing these tubes is fairly simple. Remove the lights cover, see if one of the tubes has a black end, this will usually be the bad tube. If you have a 2 or 4 light fixture, I would change them all as they could be the same age, so the others will probably go out in the near future.
If you have a straight version, grasp the tube at both ends and twist the tube 1/4 turn in either direction and the pins will slide out of the socket. Replace the tube by having the pins on the end in a 12 and 6 O’clock positions, making sure the pins slide straight up into the slots on each socket, then twist 1/4 turn until they snap into place.
If you have the U shape version, you will need to just pull down gently on both sides of the tube to remove it from the sockets and the small spring clip on the curved end.
You may also encounter a round fluorescent light tube. These have a 4 pin socket that just plugs in.