Its the lever itself not moving back, or is the brake not disengaging?
If it's just the lever, it may simply need to be lubed (you know moving parts on your bike require lube from time to time). It could also be the brake switch not releasing (it's the small black box attached to the bottom of the control assembly). There is also a return spring in the master cylinder assembly. But more likely, the plunger is hanging up if the plunder isn't returning. If that's the case you'll need a rebuild kit for the master.
Try some lube on the lever pivot. Chain lube would be fine (WD-40 is not a lube, its a cleaner).