Sliding doors are supported either at the top by rollers on which they hang, or at the bottom on rollers or tracks on which they slide. To remove either type, you generally lift them up, and then angle them out toward you. For some top-hanging doors, you may need to match the position of their rollers with a notch on the track before you can remove them.
Keep the tracks of sliding doors clean by periodically scrubbing them out (a toothbrush works well). A quick vacuum with a nozzle attachment can help prevent dirt from building up in the tracks. While you're cleaning the tracks, tighten any loose screws that are holding the tracks to the floor or wall frame. Spray penetrating oil on the rollers to keep them moving freely. A very light coat of oil on the tracks can also help.
Tracks, particularly those attached to the floor, can become bent. To fix this, take a piece of wood that's long enough to extend past the bent area on both sides, and narrow enough to fit snugly into the track. Place it in the track against the bent portion of track, and hammer the track back into place against it. (To avoid damaging the surface of the track, hammer against a piece of wood or carpet scrap placed against the track, rather than directly on the track itself.)
A sliding door track that has been bent can sometimes be fixed by hammering it back into place against a block of wood.
Doors that hang unevenly can often be adjusted using the mounting screw that's designed to hold the door level. It's probably located below a top-mounted roller or above a bottom-mounted roller. Some adjust the door's level when you turn the screw, while others are mounted in a slot. For these, it's good to have a helper. Loosen the screw so that the screw moves within the slot, and move the door up or down until it's level. Have your helper hold the door in place while you retighten the screw.
If the door isn't moving at all, again, check the mounting screws. You may need to adjust them so that they lift both edges of the door up so that they're within the bottom track, but not dragging along its surface. For a top-hanging door that doesn't have bottom tracks, check the guides that are attached to the floor with screws, usually at the center of the doorway and often on each edge of the door frame. If they've shifted, they may be impeding the door. Just loosen the screw, adjust their direction, and tighten the screw.
If none of these fixes work, have a look at the frame around the door. If it has warped to the point of moving or bending the tracks, which are now impeding the doors, you may have to replace the tracks.