I am in the exact same situation. Phone literally ground to a halt last night, so rebooted it and it got stuck in a boot loop. Phone isn't rooted, it wasn't unlocked at the time (it is now, since I've been trying to fix it for the past day) and I literally have no idea how to fix it. Although I think my situation is worse, I can't access anything else other than the bootloader...
I've tried RUU (which I can't do because I'm not S-Off and I'm on hboot 1.45)
I've tried just flashing a new recovery (but whenever I try to enter the recovery it gets stuck saying 'Entering Recovery' then the phone goes back into the boot loop)
Like literally the only thing I have access to is the bootloader, everything else sends it into a boot loop, hell I can't even format or flash imgs (apart from recovery imgs) without fastboot giving me an error message.
I fear the phone is completely ****ed, and this has come out of completely nowhere. Except now that I've unlocked the bootloader to try and fix it, it says TAMPERED and UNLOCKED which means I WONT GET A REPAIR...