Oldsmobile Intrigue Serpentine Belt Replace/Remove

I Brought The Car To A Mechanic They Replace The Serpentine Belt And The Water Pump. My Question Is Does The Serpentine Belt Have To Be On...

I Changed My Serpentine Belt But Had To Un-hook Radiator Reservoir To Do So. Now My Coolant Light Is On And My Thermostat Does Not Seem To...