Stuck in TWRP after trying to run a WCDMA test from the engineer's menu

So, I entered the engineer's menu on my LG G3 (T-Mobile: D851) by dialing 3845#*851# on my numpad, and I tried to run a WCDMA test. After doing so, my phone shut off and I was launched into TWRP (v2.7.1.0). I tried to restore my phone with an old backup, but it did not work. I'm not sure why. I've also tried factory resetting it. I simply rooted my phone when I bought it to apply some performance tweaks like saving battery by hibernating apps with Greenify, so I'm pretty ignorant when it comes to solving problems like this. What should I do?

Thanks for your help :)