Which version and edition of SQL server? Did you try to
attach the database, or did you just copy the files and hope the server would see them?
I've only ever seen this happen with a machine that was running MSDE. When the W7 upgrade ran, it did not shut MSDE down gracefully, nor did it disconnect it cleanly*. The really short version of a painfully long story: we had to roll the machine back to the last image that the client had of it with XP, then do it the right way. I had no luck salvaging the original MDF.
Please provide more information on your version of SQL and what, exactly, you've tried, and I'll try to provide more help.
*(The MDF file was left in a read-only state, and possibly corrupted. SQL 2k5 won't attach an MDF that's read-only; reinstalling MSDE and attempting to reattach also didn't work (the possible corruption).