Power seat wont move Front to back, But the back of the seat will move

Remove the seat, unbolt the drive motor and set the seat to the desired location, reinstall the motor.

The electric motors that powered your seat to make it move in those different ways has now died so what you need to do is replace your electric motors underneath your seat and it will be working properly again

There should be a lever under the front seat

Fron first principles, the reverse movement of the motor is caused by reversing power polarity to the motor. If he motor runs in one direction and not the other then it isthe switch. It is not conecting power to the reverse contact. Check.replace switch.

will or wont move foward?
you can get to the wiring block under seat. isolate wiring from motor 1 of three, (middle motor- foward back, front motor - front seat base up / down, back motor - back seat base up /down) does it work of mem. buttons? or when u lean seat foward (entry assist).

check fuse.if fuse is good check for power at seat motor.if yes replace seat motor.if no check wires at switch.if wires is not loose or broken seat motor switch could be bad.

You can move the drivers seat back by turning on the power, getting in the seat and position your feet firmly on the fron flor with your back firmly against he seatback, and then engage the switch pushing back - while pushing like hell with your feet. You can actually hear the seat drive shaft turning while the seat moves back. The principle should be the same for moving forward, but pressure would have to be aplide to the back of the seat.

Before you start beating up thing's , test the electrical circuits. Do you know anything about automotive electrical systems ? How to use a volt meter an a wiring diagram to test for B+ power to the switch an testing the ground .
Does your envoy have memory seats ? Your vehicle has four or more electric motors to control back an foward , up an down etc.. the odds of more then one motor being bad at one time are highly unlikely .
Look under the seat any trash under there keeping it from moving ?
If you have memory seats there is a control module involved in making the seats work . If it has memory seats it could have DTC'S - diagnostic trouble codes .
DTC B1735, B1740, B1745, B1750, B1755, B1760, B1815, or B1820
The seat adjuster switches are inputs to the driver and passenger seat modules. The seat modules supply the adjuster switch assemblies with a battery positive switch supply circuit and monitor the switch signal circuits. In an inactive state the switch signal circuits are floating and when a switch is pressed the signal circuit is closed to the switch power circuit within the adjuster switch assembly. When battery voltage is present on the switch signal circuit the seat module determines the switch status to be active.
DIY Blazer GMC Jimmy Power Seat Control Diagnostics and Repair Electric...

Try tapping the seat motors under the seat while operating the switch.
Sometimes this trick will cure the problem for a short while. If the seat starts to move, then you will need a motor assy. If the seats do not move, there is a 2 wire plug that supplies power to the seat motor assy. check to see if you have 12 volts to the red wire.

woman with a hammer fixed 2004 Rainer! Had same problem. just tapped rear runners with hammer and it broke loose. Works fine now!!!
Hope it works for you.