First of all I would not drive this vehicle until you have it serviced because you risk shelling the whole transmission.
Usually if there is an electronics problem, you will get a service engine light. For example if the ECU asks for 2nd gear and the TCU doesn't respond, you'll get a code. So this is very likely a mechanical problem.
The maintenance pan needs to be dropped and the transmission inspected.
In the meantime, I'd start looking for used transmissions at local salvage yards. They may be much cheaper than overhauling yours. (A brand-new transmission will be very expensive.)