The partition information became corrupted somehow. Your data is most likely there it just doesn¬ít know exactly where and what. Try this program or anything similar to it to recover data of drive F. Copy it to a working partition or external drive, then once your data is safe you can hit yes to the format to remake the table information.
I also suggest you Google for a program called Astra32, use this program to check the SMART status of your hard drive. It can tell you if your driving is going bad, which could have been the cause of the lost partition.