As above stated BACK UP your data.
Probably the MBR got hosed.
You can try BOOTSECT.EXE (This is located on the win 7 installation disk and is run from a dos command).
If that does not work try bootrec command.
Try googling "BOOTSECT.EXE" and "bootrec command" for more info on usage.
Have Not used myself, found Here: http://www.sevenforums.com/tutorials/20864-mbr-restore-windows-7-master-boot-record.html