2000 Silverado only has 3rd and reverse

[Gm1128]

I have a 2000 Silverado 4x4 that has transmission issues. The trans was rebuilt several years ago and work fine until i parked the truck for a year. Upon initially driving away from where its parked the trans does its job and automatically shifts through all gears. Once I stop and go to take off it clearly isnt in 1st gear. Best i can tell at that point the only gear operating is 3rd. Ive tried manually shifting and that does no different. Any facts, ideas or suggestions would be appreciated.

[muddkatt]

Since it was parked for a year, check to see if mice chewed the wiring harness. Shifting is controlled by solenoids.

Check fluid level also.

[davester]

As well, you might also want to get codes from the TCM, you'll need to use a better code reader than the basic ones that can only get codes from the engine (and there can be codes, even when the check-engine light is off).

[Mikebtte]

Been a While but I believe it used to be that when the transmission was not receiving power to the transmission controller it went into default and that was 3rd forward gear and reverse. I am with everyone else on check wiring and connections.