DIAGNOSIS The door lock mechanism (officially called the door strike) has either failed mechanically or isn't getting the signal from the control board to release. It can also happen if there's still water detected in the tub.
FIX First, make sure the cycle is truly complete and the washer is off. Unplug the machine from the wall for 2-3 minutes, then plug it back in. This often resets the electronics and the lock will click open. If not, check for an error code on the display that might indicate a drain problem.
IF THAT DIDN'T WORK There's a manual release. Look for a small flap or removable panel on the lower front edge of the door opening (often near the bottom right). Behind it is a cord or lever. Pulling this will mechanically disengage the lock, but only do this if the drum is not spinning and you've unplugged the machine.
HEADS UP If you use release while the tub has water in it, be ready with towels—it might drain out onto the floor. Also, if the door lock is failing repeatedly, the electronic latch assembly itself will likely need replacement.