1996 Hyundai coupe 2.0ltr automatic transmission suddenly won't go into reverse?

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Ok, so im really REALLY hoping that someone has seen this problem before or if anyone has some ideas as to what could be the cause of this problem it would be greatly appreciated! Im seriously hoping im not going to have to replace/ rebuild the transmission!! :( Ok so about 3 weeks ago I had the drive chain replaced in the engine, because it suddenly started making a strange tinny ding sound ,so I had it towed to the service center, and they said the drive chain had stretched, and broken the guards - so they replaced the drive chain, guards, rocket cover and gasket - then i drove it maybe 70km total.. and suddenly the 3rd time i got in the car to drive it, it just wont shift into Reverse - the shifter moves no problem, and when in line with D, the car engages and will start to slowly move forward ( not sure how fast its capable i didnt want to push it ), but when at R it doesnt engage and acts as if its still in neutral? I think the oil was low, but topping it up hasnt seemed to fix?