DIAGNOSIS: The dryer is likely refusing to start because the door safety switch is failing or the door isn't making good contact. The beep is a very common error alert for this.
FIX: First, open and close the door firmly a few times, pressing on the center of the door. Listen for a distinct *click* sound from the right-hand side of the door opening. If you don't hear it, the latch isn't engaging the switch. Check that the latch isn't broken and the strike plate is clean. If it seems fine, the door switch itself is a very common part to fail.
IF THAT DIDN'T WORK: Check if the control panel is locked. Hold the Control Lock button (it's on the panel) for 3 seconds to toggle it off. If that's not it, a hard power reset sometimes clears a glitch: unplug the dryer from the wall outlet for 5 minutes, then plug it back in.
HEADS UP: Don't start taking the door apart without unplugging the dryer first. That door switch is wired into the 120v circuit, even though it's a simple mechanical part.