When a hard drive is switched off, its heads are parked away from the platters, on a ramp or in a landing zone. If the drive is dropped, or if it loses power at the wrong moment, or simply through wear on a lightweight drive, the heads can come to rest on the platter surface instead. The surface is smoother than glass and the head is held to it by the same forces that make two wet panes of glass stick together. The motor tries to turn, cannot, and the pulsing current makes the beep.
The forums say to tap the drive, or spin it sharply by hand, to free the heads. Sometimes that does start the motor. It starts it with the heads dragging across the surface until they lift, which scores the platter through the tracks that hold the drive's firmware and your files. A drive that beeps and is left alone is a good job; a drive that was beeping and was tapped until it spun is a much worse one.
On the bench the drive is opened in clean air, the heads lifted from the surface with the right tools, and replaced with a matched set, because heads that have stuck are heads that cannot be trusted to fly again. Then the drive is brought up once and imaged, head by head, with whatever surface the sticking damaged read last and most carefully.