If you are using a more modern version of Rails and it uses Puma as the web server, you can run the following command to find the stuck Puma process:
ps aux | grep puma
It will result in output similar to this:
85923 100.0 0.8 2682420 131324 s004 R+ 2:54pm 3:27.92 puma 3.12.0 (tcp://0.0.0.0:3010) [my-app]
92463 0.0 0.0 2458404 1976 s008 S+ 3:09pm 0:00.00 grep puma
You want the process that is not referring to grep. In this case, the process ID is 85923.
I can then run the following command to kill that process:
kill -9 85923