It happens to everyone: at some point, the door on the shower acts up. It won't close smoothly, or you have to shove it open as if it were a 50-ton slab of rock. The causes of a stuck or malfunctioning shower door are simple, and some of them you can fix yourself. Others benefit from professional attention, and one in particular absolutely needs a professional approach. The good news is that, no matter the cause, it can be solved quickly.
If the sliding door on your shower won't move at all, it's got something jamming the rollers on top of the door. It could be that the door is off-track and a roller is causing the jam itself, or someone (a prankster housemate, for example) shoved something on top of the door to block the roller. You can remove temporary blocks yourself. If the roller appears to be the cause of the block, try lifting the door off the track and repositioning it; however, don't try to force the door in this case because you don't want to break the roller if it's really wedged in there.
If the sliding door moves but is tough to move, the rollers and track could again be out of alignment — just not jamming each other. Those you should be able to reposition. If one of the rollers looks bent or twisted, call a shower door or glass repair company. A technician may need to remove the door and fix the roller.
It's also possible that dust or dirt has built up along the track. If you can take the door off the track and clean the top with a cloth, that should help. Also look at the rollers and ensure there's nothing built up in the grooves.
For stalls that have a swinging door, some of the causes can be similar to those of sliding doors. There can be dirt or scale buildup around the latch that's preventing the door from closing properly; clean that off, and you should be good to go.
It's also possible that a hinge has bent. People sometimes place too much downward pressure on doors when they pull or push them, and that goes for both regular doors and shower doors; the pressure can bend the hinge. That's really something you should call a shower door service about; while you might be able to fix the hinge yourself, it's better not to take a chance at damaging it more.
The door may also have been installed improperly, and the flaws are just now making themselves known. That is something you definitely have to call in a shower door service for. Any fixes need to eliminate the problems caused by the first installer, and you likely won't be able to spot everything that's wrong with the original installation. A glass shower door services technician can inspect the whole setup and fix the problems.