This problem happened already some times to me. It is fixed with a reboot, but I am wondering if there is a faster solution.
Windows in File Manager look like this:
My nautilus doesn't used to respond So for restarting I used nautilus -q or killall nautilus
How to restart nautilus without logging out?
But both the commands didn't worked for me
So I killed the process manually and it worked for me without restarting my system
ps -ef | grep nautilus
kill -9 ID
you can try both to see if it helps whenever you are facing the issue again