It's highly likely that the interior door handle cable broke or became dislodged. You should be able to tell, as there won't be any resistance on the handle when you're trying to open it from the inside. Fairly easy fix if the cable simply became dislodged, or you may need to replace the handle if the retainer clip broke. The door card removes via 2 screws behind a rubber cover in the armrest, and then you can access the cable and determine what happened. The cable has a barrel end, which clips into the handle with a retainer.