Loss of Lightroom 4 catalog

i have lost lightroom 4 catalog. caused loose connection external drive keep recent photos. loose connection caused lightroom crash message 'fatal error'. lightroom 4 catalog backup kept on main c drive in pictures. had backed day previously. don't understand why whole backup file has apparently been deleted. have done search see if elsewhere on drive can't find it. when try open lightroom message 'upgrade catalog file lightroom 3 use lightroom 4.

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