Haven't disconnected any drives at all. The dual boots are both Windows 7 but a different bit (32 & 64) the 64-bit boot is the one gone.Did you disconnect any drives, at all?
Did you recently happen to upgrade to a newer build of Windows? Is your dual boot two versions of Windows 7 or Windows 7 and something else?
Did you make any changes to any configuration settings in Windows or in the BIOS recently? Changed any hardware?