There are a couple of styles of connectors for keyboards, one is the Zero Insertions Force (ZIF) that has a bail. which may be missing if the ribbon cable came loose on its own. The other style is the Low Insertion Force (LIF) that you just carefully push the ribbon cable in and make sure that the cable is inserted evenly.
If the connector is a ZIF connector and the bail is missing then try holding the ribbon cable by inserting a piece of thin card stock into the connector on the opposite side of the contacts. Use a piece of tape to hold the card stock in place.