It's possible you have tripped up itunes in some new way. Here I know to try another user account but some folk can't try such things.
There is something new in your situation so let Apple answer this one.
I have 14 iPad's
Sync to 1 Windows 7 machine.
Everytime I close iTunes and reopen, the library is empty and says I have synced to another machine. It does this even without disconnecting the iPad from the PC and with all of my iPads.
I only load Apps in iTunes, no music, videos, pictures, or etc...
What should I check first?