CSPROJ files are XML - so the error is relating to the header for the XML file, which should look something like this:
="1.0"="utf-8"
<Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<PropertyGroup>
...
If it doesn't - and you can check using a text editor like Notepad - then it's corrupt, and that is what is causing the error.
How it became corrupt, we have no idea - it may be a hardware fault, it maybe a software glitch: I'd strongly recommend running an integrity test on your hard drive(s) to check them for problems as such do not get better, they get worse and generally quickly. Very quickly.
Check your backup (and three months is an excessive time between backups) and look at the file then. If that doesn't look right either, but your HDD looks ok, I suggest creating a new solution and importing all the existing source files to that and see if that works.