Booting to TWRP recovery, you should be able to copy /sdcard content (your files) to a computer using MTP.
If your phone was encrypted and TWRP is not asking for password/scheme, unfortunately you can’t backup /data.
If not encrypted or decrypted after asking for a password/scheme, then you may try to take a TWRP backup ([HOWTO] Backup and Restore Data on Android Devices).
However, please be aware that restoring parts of /data (like contacts, SMS, …) is very time-consuming and painful 
You may try to go back to 0.18 at a first step : push the ROM image to /sdcard, then use TWRP Install button …