Connect the phone to your PC.
Complete all these steps:
1. Download a ROM from your PC and transfer to your SD Card (Your phone will be used as the USB storage)
2. Rename file to update.zip
3. After transferring, remove your phone and reboot, it will go back into clockwork recovery.
4. Install update.zip
5. Reboot
This exact thing happened to me twice.
how can I transfer files when I can't boot to mount it?
Connect your phone to the PC and see if your PC can read the phone as a drive. If so, we are in business. All you need to be able to do is just get to the boot screen.