At some point she leaves Canterbury Commons, with the family dog Myles, in search of aid; medicine from the Citadel and some form of assistance from Reilly's Rangers. However, she never came back, spurring her brother Manny to form a search party whose objective is to track her down. This brings about the unmarked quest Searching for Cheryl.
Unfortunately, she is never actually found, with most of the members making up the search party sent after her also coming to various unfortunate ends, including her brother.
Cheryl is only mentioned in Fallout 3.