lucy24 - 11:43 am on Feb 25, 2013 (gmt 0)
Obvious first step: What if anything do your logs say? Look at both access logs and error logs; there's a good bit of overlap but each will say some things that the other doesn't.
Do you know any other sites that live on the same server? Are they behaving normally? Is it your own server or shared hosting?
I assume you haven't changed anything-- even something completely unrelated-- in the last few days, or you'd have said so.