2001 GMC Sierra engine light on

Hi my question is this, my truck started to hesitate like low voltage or idling low then parked the vechile. Came back to start it, had a difficult time but made it home around 2 blocks. Next day tried to start it bt would not crank over. Thought it might be the altenator and was told this was not the cause. Towed it from there to a mechanic and for 2 days has'nt figured it out yet. Don't know to say but ay help wouldbe apreciated. Thanks for your hellp, dan.