I found this answer online on another guys post from a GM Master Tech.
If it only has reverse, and it wont even try to engage into any drive gears, Then Yes the issue is internal (bad forward clutch pack,or broken shift valve in the valve body,Or one way clutch sprag) The Transmission will have to be removed and dissembled for inspection.
Not good news for sure. Hopefully you know a good rebuild shop.
Stock 2003 3.4 alero. Pioneer 3400ub. Pioneer 4 way 6x9s. Pioneer 3 way 4x6s. 800 watt JVC amp powering 2 12" MTX Audio terminators.