Transmission only engages in reverse

Typically, when a vehicle has lost everything except reverse, it is most likely a problem with the transmission or gear shifter. With that in mind, and since you already checked the fluid, the next step would be to inspect the shifter linkage to make sure it is actually moving the shifter on the transmission. Your vehicle has a cable which actually does the shifting and it could be damaged, stretched, or need adjusted. Check that first. I attached a picture of what it will look like.

Thank you very much.

Let me know what you find.