If you can enter fastboot, have you tried to reinstall the TWRP and then a ROM?
If you can enter fastboot, you can install the ROM by adb commands:
Download the zip of the ROM.
Connect the mobile to the PC in TWRP Recovery mode.
In TWRP Recovery, give settings, ADB Sideload.
Open a command console, type ADB Sideload and name the ROM.zip.