Error 404

This morning I worked on one of my sites without a problem. I then went to another site and received the error 404 message. For this to happen seemed strange since I just worked on a site and now I can't get past the login. Is there something happening with the server? I use Google Chrome most of the time and did this morning. I also tried using Firefox to see if that would make a difference and nothing. I hope this is resolved within the next 12 hours as I have a lot of work for my clients that need to be done.