My drone won’t take off because one of the propellers is not spinning. But it’s not electrical or software, because when the drone is off, the bottom circle (to which the two replaceable wing are attached) won’t spin (on the other 3 it’s fine). This is strange because the drone worked fine just an hour before so I find it hard to believe there is anything stuck there like dust or something like that. Any ideas?

The silver bit that rotates is called the rotor.
Where were you flying in the last flight,  anywhere dusty or sandy?
A guess would be that you have sand or grit causing a jam. I would turn the drone upside done and, using ONLY YOUR FINGERS, try twisting the rotor clockwise and anti-clockwise whilst tapping it with a knuckle or wooden spoon/stick. START by twisting the motor towards the trailing edge of the propeller blades. Ideally do this over a large white sheet of cloth or paper, so that you can see if anything drops out.

If that doesn't work turn the drone the correct way up and do the same thing i.e. twist and tap the rotor.

Hi, Alec_P. Thanks for reaching out, and we're sorry you're experiencing such concerns. In addition to the suggestions posted above, kindly check whether there are any error messages shown on the DJI Fly app. If possible, please post a short video showing the issue for further checking. Thank you.

Can’t send the image or video for some reason

But it says motor stuck, check for objects blocking motor

Cheapest option first, maybe blast it with a can of electrical cleaner?  If the propeller is clean and securely attached, there might be an issue with the motor itself. Motors can sometimes fail due to various reasons. In this case, you may need to contact DJI customer support or a professional drone repair service to diagnose and repair the motor.

What is electrical cleaner? Never heard of that

or just use an air compressor.

Alec_P Posted at 5-26 14:37
What is electrical cleaner? Never heard of that

It's a solvent and I would be VERY wary of spraying the motor with a solvent cleaner, you might destroy the lubrication in the bearings etc.
I would suggest the can of compressed air idea is much safer than the electrical cleaner idea. Air  straight from a compresseor might be too forceful so be careful if you try that.

The whole thing still seems strange after the drone flew with no problems a day before… in clear conditions no dust no nothing. But the drone was crashed a couple months back so that must have something to do with it

I would doubt month's old damage is causing the problem unless it caused a crack etc. that has grown to become a full break. The clearance between the motor's rotor and the motor stator poles is very small, it does not take something large to cause a jam.

I would suggest you switch syncing of and then attempt a flight/motor-start. Then copy both the .txt and .DAT flight log to comupter. Upload the .txt to
https://www.phantomhelp.com/logviewer/upload/
and post the URL here.
Upload the .DAT to a file hosting site, make the page public and post a link here.

From memory the csv of the .tzt shows the motor current.
Assuming those figure are not 'window dressing' I don't remember if it is per individual motor or total but the DAT might show individual motor currents. If either log dose show individual motor currents then I think a jammed motor will draw an exceptionally large current before system safeguard hopefully shut down the relevant ESC. Such a large current draw might even show up in the total current reading.
If the fault is a circuit break then I would think that the afflicted motor would show 0, zero, current. Again that might show up in an unusually low total current