stock restore won't boot twrp

I've come up a bit of a croper.

I am bootloader unlocked, after which I flashed the latest TWRP recovery. After this, the first thing I did was back up my stock ROM, as it was.

After being on a custom ROM for a while, I wanted to go back for a bit just to see if I could feel any difference.

So I wiped the custom ROM, and restored my stock backup image.

Which wouldn't boot.

I've noticed there's a pattern when you turn on the phone. "HTC powered by Android" splash screen, with a dim display. The display brightens briefly, and then dims again before loading the ROM. Reminds me of the PS1 swap disc trick pattern. It speeds up, slows down, speeds up, swap, spins down etc.

Anyway, after restoring the stock image, it got as far as the brighter display before getting stuck there. Have to hold power + vol- for 15 seconds (going by the touch key flashes) when I can then get to download >boot loader > recovery.

Anyone have any ideas why it won't boot? I am s-on, but custom roms install fine regardless of this.

from xda-developers http://ift.tt/2cIVyBB
via IFTTT