The first thing to try is to run an upgrade install. Just go to yoursite.com/install (you’ll have to re-upload the install folder if it’s gone).
As long as you check the upgrade checkbox and make sure not to install the sample site or any add-ons, you shouldn’t lose any work.
If it doesn’t work, maybe it will produce a helpful error message.
I notice that the source is now:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=Content-Type content="text/html; charset=windows-1252"></HEAD>
<BODY></BODY></HTML>
The charset is unusual. Are you on a Windows server? If so, you should be asking these questions in the Windows Hosting forum.