C# - Class File Crashed

If you have a backup, I would restore from there.
If you don't, then the chances are you will have to start the class again pretty much from scratch.
 

If your system terminated unexpectedly, it sounds like it was writing the class file when it failed - and the results are a mangled or unreadable file. In all probability, the data in teh file is just rubbish, and cannot be trusted.
 
Try making a copy of your project folder, and opening VS normally - check if the class file is even slightly readable, or VS may offer to restore a temporary file. If it does, go for it and hope you haven't lost too much work.
 
And if you don't have a backup, you have just discovered why I have an automated system doing incremental backups of my files each and very hour...