No forward and reverse gears

I have a C4 rebuilt in April, 2007 and now has about 15,000 miles on it. The repair shop used all new parts except for the casing because before, the torque converter had disintegrated and sent metal shavings through the whole thing. The car was driving fine and suddenly stalled, when I started it again and put it in drive, it wouldn't move except for rolling back. The car has no drive in all three forward gears and reverse. The same thing had happened the day I got it back from the shop when it was a fresh rebuild after driving only fourteen miles, the shop had fixed the problem under the warranty and it was determined that the cooling lines were not cleaned out before hooking them up to the new trans cooler and the transmission, and as a result sent metal shavings through it again and messed up the valve body. The tranny has had a small leak since July, 2007 when I picked it up out of the tow yard and they had brouhgt it out on a forklift because they left the keys in and the ignition on the whole time it was in there and wore the battery dead. I have been keeping up on maintaining the fluid level. Once in awhile, it would take a few seconds after I first started the car for it to engage in either drive or reverse. I have checked the fluid level and the shift linkage and everything seems good. The fluid is not discolored and does not smell burnt. I have not readjusted the bands or dropped the pan and looked at the valve body. The warranty is now expired and probably have a $1200.00 piece of junk for a tranny, but hope not. If anyone has any opinions on what the problem may be, I am looking forward to hear anything that might be an easy fix. Thank you.