If you are on shared hosting, the only thing you can do is keep things as secure as possible. If someone else on the same server has a vunrability, then it can be exploited and affect others on the same server. The only thing you can do really is to be secure, take regular backups, and if something goes wrong, don't panic, seek advice.
Well this is defintitely a BIG lesson in security for me. It kinda sucks if it's because of another site on the same server. I guess there's not much you can do about that if that is the case. I had two admin accounts set up. One of those passwords was extremely easy as he never changed it after set up. I'm thinking this could be the reason it happened but I would definitely be interested in any kind of extra security I can install on the board.