DIAGNOSIS The power light is on, which means the unit has electricity, but the heating element isn't engaging. The most likely culprit is a failed or loose connection in the thermostat or the heating element itself.
FIX Unplug the grill and let it cool completely. Remove the drip tray and the grill plates. Look at the underside of the grill head where the heating element is housed. Check for any obvious, loose wire connections (usually white or black wires) going to the thermostat or element. If you see one, you can try to carefully push it back on its terminal. If everything looks connected, the internal thermostat has probably failed and needs replacement.
IF THAT DIDN'T WORK The heating element itself could be burned out. You can test for continuity with a multimeter if you have one, but generally, if the connections are solid and it still doesn't heat, the entire heating assembly needs to be replaced. For the GR2144P, this is a common end-of-life failure.
HEADS UP This is almost always an internal electrical fault. If you're not comfortable opening up appliances, it's time for a new grill. These aren't really designed for deep component-level repair by users.