Without more detail, nobody can begin to guess....
What are the source files?
Where did the source files come from?
Where were the source files when you started the project?
Where are the source files now?
What did the red message say?
What steps did you take to save the project?
How long did you work on the project?
What are your "Auto Save" settings in Preferences?
Can you try again and post a screen shot of the red message?
What computer operating system are you using?
How much disk space do you have?
How much memory do you have?
And, anything you can think of that might be relevant?