Soft contact lenses are partially or even mostly made of water. When not in use, it is important to keep them hydrated by storing them in contact lens solution.
Sometimes a lens can get stuck on the underside of the lens case lid or otherwise accidentally dry out. It’s usually possible to revive and use the lens again. As lenses break very easily when they are dry, it’s important not to touch or try to remove a dry lens that is stuck to a surface.
Soften the lens with a multiuse solution and wait a few minutes before attempting to remove it. Place the lens back in its case, cover it with solution, and wait at least an hour before using it again.
Read here for tips on what you should do if you forget to take out the lenses before going to sleep.