Diagnosis
A non-stop beeping chorus is usually the control board's way of signaling an error it can't display. The most common trigger is a faulty door switch—a safety component that tells the microwave the door is securely latched.
How to Fix It
First, try a hard reset. 1) Unplug the microwave from the wall. 2) Wait two full minutes. 3) Plug it back in. This can clear a temporary glitch. If the beeping starts again immediately, focus on the door. 4) Open and close the door firmly several times, listening for a distinct 'click' of the latch engaging. Sometimes debris on the latch hook can prevent a full close.
If That Didn't Work
If the beeping persists, one of the door switches is likely stuck open or closed. These are mechanical switches behind the door latch area. Diagnosing and replacing them requires taking off the outer cabinet, which involves dealing with high-voltage components. This is a job for a qualified technician.
Heads Up
Do not try to use the microwave while it's beeping incessantly. The safety system is confused, and it may not start, or worse, could operate with the door perceived as open. Unplugging it is the safe move until it's fixed.