How do I restart this phone stuck in DFU mode?

Plug it in to charge for 20 to 30 minutes just to make sure it has enough charge for the next bit. It will likely restart several times during this process.

Then: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the recovery mode screen. You may need to try it several times before it sticks.

If after several tries, it does not go into recovery mode, and keeps rebooting, then it may be a hardware failure, and you will need to send the iPhone in to Apple for repair.

If it does go into recovery mode, then you'll need to plug it in to an computer running a current version of iTunes, and try to restore it again.

Plug it into your computer and restore iOS using iTunes.

What error message are you getting exactly?

Wait, is it in DFU mode, or is it in a boot loop? They are two different things, and the way to fix it, is very very different.

You're welcome. Glad it worked out.