Diagnosis
The turntable motor has likely failed. It's a small, geared motor underneath the floor of the microwave cavity that turns the hub the glass plate sits on.
How to Fix It
Unplug the microwave. Remove the glass tray and roller ring. You'll see a plastic hub in the center. Underneath that, accessible from below after removing the bottom panel, is the turntable motor. Disconnect its wiring harness, remove the mounting screws, and swap in a new motor. Make sure the new motor's drive coupler aligns with the hub.
If That Didn't Work
Before replacing the motor, double-check the simple stuff. Is the glass tray sitting correctly on the roller ring? Is the roller ring cracked or broken? A misaligned tray can jam and stop the motor. Also, ensure no food debris has fallen and jammed the hub mechanism.
Heads Up
When you order the replacement motor, get the correct one for the R-1874T. The direction of rotation and the speed matter. Installing one that spins the wrong way or too fast can throw the tray.