There are several reasons your shower may not be draining. Most likely it is because there is a clog in the plumbing that is holding up the water. The clog can be caused by a blockage in the drain located near the drain opening, farther down in the plumbing system, or at the end of the plumbing system near the main line or septic.
Other issues, like freezing pipes or roots breaking the pipes, can also be causing your shower drain to clog.
Identifying the primary cause of the clog will help you with your solution. For example, if you find that your roots have broken through and split your pipe, then pouring cleaning solution down your drain won’t solve the problem and you’ll continue to have a shower backup.