There's two modes you can get into while the phone is initially starting.

If you hold the volume down button, and the power button while the phone is starting, it will enter bootloader mode.

If you hold volume down + power + ok button (center of the d-pad) while the phone is starting, it will hard reset.

So, try that first. Pull the battery out, put it back in then...

Hold vol down, and the center of the d-pad... then hold the power button, you will get a grey screen asking if you want to reset the phone, push vol up to do so, then vol up again to restart it.

If that doesn't make it boot, pull the battery again, put it back in then...

Only hold vol down and the power button, you'll get the tri-color bootloader screen and you can try re-flashing the rom again from there.