Drupal site with big problems! Can anyone help?

My Drupal 6 website has recently developed some big problems and I can't seem to find a way to even debug them, let alone fix them. Not sure if I may have been hacked?

At the moment, the biggest issue is that when I try to log into the site as admin, I no longer get redirected to the admin menu but instead to "/node". There seems to be no way to get to the admin menu right now.

Before that happened, I was trying to debug another problem with concertina expanders in the editor not working - ie, where I would normally be able to click on a heading in the content editor to expand the function (like the "Vocabularies" expander here), these links are no longer clickable. Also, the WYSIWYG editor itself has disappeared (which may just mean it's defaulting to a user type where WYSIWYG is disabled).

My first goal is just to be able to log in again as admin. Anyone got any clues about that? I have a backup of the site, but I will lose quite a bit of forum content if I use that.

Post edit: By the way, a the moment if I go to the home page of the site I see the "site maintenance" message, while if I go to the /admin URI, the site still seems to be online, but clicking any of the links gets me nowhere. The URI loads in the browser address window, but only the home page gets displayed.