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That could be one of several problems. The easiest to fix would be, if you have left the microwave door open.
But it is more likely to be a simple malfunction of the control circuitry. This means, it may well need servicing, or replacing.
However, first tell me, how many beeps are sounding? if two beeps, that's the steam sensor signal... and, it may be falsely sensing, if that has kept on and on and on, sounding.
Also if you need advice as to where to get the microwave serviced, or how to have a house call come to fix it, let me know your local zip code, please.