I regularly get this problem. The "Find in files" function of VisualStudio (2008!) stops working. Whatever I search for it always says "No files were found to look in. Find was stopped in progress". It is very annoying.
Since I have only found one other blog that mentions it and gives a solution (here - thank you!), I'm going to reproduce it.
Solution: press Control + Scroll Lock (yes, it doesn't make much sens but it works).