boot camp is no longer showing on start up when pushing alt option.

After installing boot camp and windows 7 professional everything was waorking well changed my start up pref to Mac and usees option Alt at start up to select windows. Now however after a back up to external drive and running disk encyption. I'm no longer seeing boot camp as a bootable potion on start up. I atempted chaning to boot camp on start uo but it ask for a bootable disk. Any help??