The 4L60E needs power & signals supplied by the PCM to command the 2 shift solenoids off & on. When you disconnect the trans connector, or pull the fuse for the circuit that provides power, when put in "D" the trans will only have 3rd gear because BOTH shift solenoids are off. If in this state you pull the shift lever down into L1, the trans will actually go into 2nd due to the manual valve providing pressure to the circuit.
Maycko, you did not indicate if there were any codes stored when this vehicle came in.. Were there? If so, please list them for us.
If it matters, I just had a '96 4L60E come in last week with a similar problem to yours. It had REVERSE only. No forward movement. The customer stated she was accelerating from a green light, heard a pop, and then the trans was in neutral. She tried all manual FORWARD positions but the vehicle would not move. She then tried reverse and it worked so she backed off the road into a parking lot. When I tore that transmission down, I found that the bearing in the low planet set had gone out, then the splined section of the low planet that mates to the output shaft separated from the rest of the planet. I rebuilt that trans with a new low planet set, bushing kit, over-haul kit, thrust washer kit, bearing kit, torque converter, and she picked it up the next day happy as can be. Her pan also had very little debris in it, just a smattering of gray material on the magnet.
If you have no codes indicating problems with electrical inside the trans (and they weren't erased from a dead battery, or battery disconnected) then I would have to say the problem is INSIDE the trans.