* Reverse-only band engagement: The internal bands or clutches within the automatic transmission that control forward gears have likely failed or are severely worn. The reverse band may still be functional, hence the ability to move backward. This is the most common cause for this symptom.
* Shift linkage or cable problem (less likely): While less probable given the specific symptom, a severely damaged or disconnected shift linkage or cable *could* theoretically allow reverse but block forward gears. However, it's more likely to result in inability to select *any* gear, rather than just forward gears.
* Low or contaminated transmission fluid: Extremely low fluid level or severely contaminated fluid can cause internal damage and erratic shifting, but usually leads to more general problems than just forward gear failure.
* Internal transmission damage: More significant internal damage, such as broken gears or a severely damaged planetary gearset, is also possible.
In short: You need professional help. Attempting to diagnose or fix this yourself is likely to lead to further damage and significant repair costs. Towing the vehicle to a qualified transmission specialist or mechanic is strongly recommended. Driving it in reverse for any extended period is also not advisable as this will further damage the transmission.
