Phone won't boot and won't enter recovery, even after reflashing TWRP
So, I got it working again. The method was to not flash using fastboot.
I actually booted the recovery remotely with

fastboot boot recovery.img


Then flashed cyanogenmod (the system) and gapps from the external SD card.
On reboot, the system updated itself and it's working again.

And I can boot into cyanogen recovery mode (but not TWRP, as before).

Seems that it's fixed! :-)