BuddyPress.org

edit/ beaten to it check that link as it will have the best advice.

Simple security through obscurity firstly; don’t use the name Admin change it to something else ensure a really strong password, was yours strong before?

this is an example of a strong password and would take a long time to brute force:

aktE49!-Fr2?*

If you are not using a mix of alpha numeric characters, upper and lower case, and symbols then your password is essentially weak, and password strength indicators are not necessarily that reliable if they simply say strong because a character is entered more than 8 x.

I would check carefully all folders for odd files, check server logs if possible, change any other passwords especially your DB, FTP.

Also chek the WP forums and codex for further information that may be available.