How to fix a Frozen Cursor on iMac

Firstly, try a basic step: restarting your iMac. Press and hold the power button until your iMac shuts down completely. Once it's off, wait a few seconds, then press the power button again to turn it back on.

If that doesn't resolve the issue, move on to resetting the System Management Controller (SMC). This can often resolve issues related to power management and hardware functionalities. The process for resetting the SMC varies depending on the model of your iMac. Since you have an iMac 21.5" 4K running macOS 10.13, here's how you can do it:

1. Shut down your iMac.

2. Unplug the power cord from the back of your iMac and wait for at least 15 seconds.

3. Plug the power cord back in.

4. Wait for another 5 seconds, then turn on your iMac by pressing the power button.

After resetting the SMC, check if the cursor is still frozen. If the issue persists, we can try resetting the NVRAM (non-volatile random-access memory) or PRAM (parameter random-access memory). Here's how:

1. Shut down your iMac.

2. Turn it on and immediately press and hold the Option, Command, P, and R keys simultaneously.

3. Keep holding these keys for about 20 seconds, or until you hear the startup sound for the second time.

4. Release the keys and let your iMac continue starting up.

Once your iMac has restarted, see if the cursor is responsive again. If it's still frozen, it could indicate a deeper software or hardware issue. At this point, I'd recommend reaching out to Official Apple Support or visiting an Apple Store for further assistance. They can provide personalized troubleshooting steps or arrange a repair if necessary.