Premature end of script headers: Drupal Error

After moving servers today one of my accounts stopped working. The other 90 odd websites were fine, but one just threw up a blank white page.

What to do??

First check the error logs to see what the reason is, and this is what the apache error logs told me. Premature end of script headers: index.php. Of course don't forget that there are lots of error logs and sometimes a few sets in a few place - make sure you check the right ones.