some browsers login file not found blank

Hi,
from one day to the next I was unable to access my login page. The login page comes up fine with IE, but not firefox or chrome.I get page not found error 404. I am used to working with firefox and have had to switch to access my dashboard. Re-naming folders, switching themes etc does not work, I am a total newbie with wp and don’t want to muck things up, have no idea how to change code on my own, do I have to import to my computer files through my ftp, or do I need to do something in the database? My site is fine, just not the login.. Help!?