When the spindle is off can you spin the collet/shaft freely by hand? Sounds like the bearings could have bound up?

I know when I have a clogged cutter which stalls in the work the motor coils can still make a sound like they are spinning up, even though the cutter is not moving. Freeing the cutter sorts it out.

Also worth removing the plug on top of the spindle and checking continuity between different pins on the spindle side (windings). Should be a couple of ohms across a winding, not a dead short, for example.