System UI has stopped--Stock OS

Earlier today, I got an error upon starting up my Samsung Galaxy S5 (T-Mobile) that says "Unfortunately, System UI has stopped." My background has been turned to black and none of the buttons work properly. The phone IS rooted (and has been for about a week), but runs on stock OS (5.0 Lollipop) and stock recovery. I have never flashed a rom on this phone. I already tried restoring from a Titanium Backup I made yesterday before this error ever showed up, but it did nothing. Perhaps it's relevant that this error first happened after I booted the phone in recovery mode (to see what stock recovery looked like), then selected "reboot phone." I also already tried wiping the cache partition, wiping the System UI cache, and rebooting is Safe Mode. None of these worked. I thought unrooting the phone might help, but I accidentally uninstalled SuperSU without removing the root. With the System UI down, I can't install any apps at all, such as SuperSU again or a file explorer that would allow me to remove the "su" folder in /system/bin/. Now I have no way of unrooting, short of wiping the phone. I DO NOT want to do that. How do I get my phone back to normal?

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