A crash is the result of a bug or fatal error in a map or by the server itself.
When a crash happens, all players will freeze, and your client will disconnect itself after 25 second if the server doesn't respond during that time. The server will also dissapear from the servers list.
A crash is NOT dangerous for your computer if it comes from the server. It CAN be dangerous if the crash happened from your computer. A crash happening from a client (your or any other players' computer) will not interfere with the server.
Many servers have an automatic system which restarts the server after a crash and will start with the default map (the first map on the maplist).
If you wish to reconnect after a crash, you should just wait a few seconds and the server will soon be in your servers list again.
A fatal error prevents the map to be in a final/stable state unless fixed.