how do I get my liftgate to work

There may be an issue with the handle or linkage. This could also be a door lock actuator. Some disassembly and diagnosis is required to figure out exactly what's going on. If you prefer to take it in, use this link to find a good shop near you. https://repairpal.com/repair-shops Alex

From back seat - look to see if anything is caught in the rear gate that may have it jammed. IF that is the case - the gate will have to be FORCED open from inside.