The root is gone topic

I just installed a custom ROM on my G2 for the first time, I installed CM12. After the phone booted up I checked in the root checker app if the root still exists and it told me that the phone is no longer rooted.
Is there a guide on how to root again when the phone is on lolipop or restore everything to how it was before? I backed up everything before I wiped the phone with TWRP.
Thanks very much!