A player who gets stuck in the game/map is a player that cannot move as a result of a bug or more commonly, a glitch.
Generally, this happens when a door or a wall instantly appears out of nowhere, and on the moment it appeared you were on its location in the map (an example is in ze_Paranoid with the walls continuously being enabled and disabled).
You can also get stuck in physical props. Try shooting them away from you in order to be free.
Some maps also tend to stuck players inside displacements, mainly at the ceiling (for example in caves). Avoid jumping at such areas if possible to prevent getting stuck into the ceiling. This counts for displacements only!
If there isn't any admin on the server, and you can absolutely not move, the only way to get unstuck is by using the ZR command either !zstuck or better, !zspawn or !ztele. The !zstuck command is very buggy though, and is arguably non-functional on any server by now. !zspawn and !ztele will always teleport you back to spawn, but sometimes it is disabled by the server. If both options are disabled, there is nothing you can do (just wait out the round if you don't want to lose your score).