DIAGNOSIS The grill is stuck in the initial "prime" or "ignition" phase. This usually means the hot rod igniter isn't working, so no pellets are catching fire to bring the temp up.
FIX Unplug the grill and let it cool completely. Remove the grates, heat deflector, and drip tray. You'll see the hot rod—a metal probe near the burn pot. Check if it's broken or coated in hard ash. You can clean it gently with sandpaper, but if the element inside is broken (no continuity with a multimeter), you need to replace the hot rod assembly.
IF THAT DIDN'T WORK The problem might be the other way around: the fire pot is so full of fine ash it's smothering the igniter. Vacuum out the entire fire pot and the area around it thoroughly.
HEADS UP Always run the grill on high for 10-15 minutes after a cook to burn off residual pellets. Letting it shut down with a full pot is a surefire way to get ash buildup that causes this.