I changed my theme yesterday and now theres nothing there, just a white screen. I tried going back to change it to another but nothing. can any help?

the site ishttp://iluvthe80s.co.uk

OK, two things to look at here. If you are using the autologin hack, try to directly reach admin.php.

First the user.php blankpage. I think it is the theme issue, but also check for extra white space in mainfile.php.

No for the theme issue if you cannot reach admin.php

Take the currently used theme directory and through ftp rename it to something else. Next take a known working theme, perhaps the default theme, rename the directory to the one currently registered in the preferences. Once you gain access back to the site you can reset your themes and return the naming back again.

I had this happen to me once, it was due to uploading an extra folder for the theme directory created by the archive (zip file.)

HTH.

Error in module or block.
Go to admin and check the last thing you added.
Somewhere there is the bug.

tried, says Im not logged in so can't get access