Did you try to start a mode when u came to the server? With like /votemode for example. For me it's usually a black screen when no game mode is running.
I had the same problem, I just tried again and it worked.
Edit: I did that when I already had an account. Are you unable to create an account? Try to register on mtabeta.com
I dunno if you can change it in MTASA, if not, open up single player, and when you come to the screen where you see Start Game and all that click on options, then click on Display options, Advanced, and in there you can change your resolution.