I had my 00 eclipse gt automatic do this. took it to mr transmition and they cleared codes that popped up and it stopped. if i drove forward for a while and warmed the car up, then reverse would work. hope this helps.
If you are stopped, it is the person whose car backed into you, although this could be up for debate, so hopefully there will be witnesses if not it is a matter for the insurances for a no-fault decision.
Its probably the actual transmission itself. It happened to my moms 99 Kia sephia and then it just started working for a few months than the tranny just stopped working all together so I would suggest getting it checked out