I finally got the tailgate open after 30min. by turning the LF door key in the cylinder back and forth with someone holding the tailgate handle and pushing the tailgate release button. Removed the inside tailgate panel and inspected everything. With tailgate open, observed the cylinder lock when remote was pushed or LF door locked, but would not unlock. Tried to replace the tailgate lock assy., but no luck. Opened up inside trim and inspected the wiring, but found no broken wires. Will now try to replace the LF Door lock assy. to see if by chance this will solve the problem. We bought the car used 18months ago and all doors,tailgate worked fine with the LF door key, except the gas door, which was broken. The remote was missing, so we bought a new one from the local Volvo dealer and had it programed accordingly. All doors,tailgate worked fine. Since the wire harness seems to be intact, and if the LF door assy will not solve the problem, where should I look next ?

I had some problems like this and it all stemmed from wire harness damage at the LH tailgate hinge - a Volvo trademark. Unfortunately, in order to examine the wiring, you first have to get the tailgate open. You might try looking for a blown fuse in the rear fuse panel and hope you find one that allows the tailgate to unlock. If not, there is some discussion here about how to manually unlock the tailgate from inside the car - I've never tried this:

http://www.volvoxc.com/forums/showthread.php?8666-Rear-hatch-locked-shut

Here is a description of what I went through to repair the wiring:

http://www.matthewsvolvosite.com/forums/viewtopic.php?f=9&t=47882


Does you gas cap door still open?
Are you sure there isn't a place by the license plate lights to insert a key? (Sometimes has a rubber cover on it)