I don't know what button(s) I pushed, but suddenly I have a command line prompt and I cannot make it go away. Any ideas? For now I will reboot the game but I would like to know how to fix it next time.

You probably hit the key just above Tab.

the "tilde" key... ~ .... opens/closes the console.

Just for the record, as tzt implied, it's not always the tilde key on all keyboards (e.g. my UK layout), but AFAIK it is always the one above TAB... perhaps that seems rather pedantic, but it's caused a fair bit of confusion before!