HVAC fan turned off by itself with associated burning smell

Question

I'm in the PNW and we're dealing with lots of wildfire smoke. Accordingly, I've been running the fan on the HVAC nearly continuously for the last 4 days so that air will go through my electronic filter attached to the system. Last night, I woke up to go to the bathroom and on my way there noticed a faint burning smell coming from the return air vents in the ceiling. I also noticed that even though my Google Nest expected the fan to still be running, it was not blowing air out of the registers.
I turned off the fan and left it off for the rest of the night. This morning, the fan (by itself) or the AC (with the fan) seems to be working correctly. What might have happened here, is it a concern, and is there anything I should do in response?
Edit: Added a picture of the register below the unit I blocked to help with discussion in comments.

Ed Beal · Accepted Answer

Your fan was not running and a hot smell points to a thermal overload in your fan motor tripping.
Why would this happen? The lubricant in the motor has dried out and the bearings are getting stiff if a belt drive it could also include the bearings on the fan itself.
Can it be fixed: Usually yes
The fan motor usually has ports over the ends that lead to the bearings. I use 30 weight motor oil but any motor oil will usually help.
Most motors only take about 20 drops of oil if the tube or shaft fills up run the motor for a while and check it later,  if you can get 10-20 Drops in let it run and the problem may go away for a year or longer but adding a few drops every 6 months can extend the motor life by decades.
The reason the motor was not running it overloaded due to lack of lubricant and there is a thermal snap switch in the motor that had not cooled down yet. I have done this little trick hundreds of times and brought motors back to life.
Don’t forget to wash your grids I can’t believe how bad the smoke is this year, the grids will be more efficient when clean.
They said they expect containment by October 31 so we may have this mess for a while.

HVAC fan turned off by itself with associated burning smell

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