I have a 2004 Buick Rainier am needing help on what's wrong. It starts but won't go forward or in reverse?
Answer: As with most all of the newer vehicles today, the transmission has shift solenoids.
It may be a problem with one of them. You will need to get the computer scanned for codes. There may be transmission related codes that would point to a bad solenoid.
Another thing you can do is go to the local parts store and buy a bottle of transmission additive. Put that in the transmission, drive it for a while as the instruction on the bottle say, then take it to a local shop and have them do a fluid exchange and filter change. The additive can help remove deposits and varnish and free up sticking valves. Sticking valves in the trans can cause the problem you are having.