I've seen quite a few threads here about phones stuck on a black screen, white screen or the HTC screen. In most cases a simple battery pull and reboot fixes this, if not, this where a recovery comes in very handy. If you flash a rom and the phone then fails to boot, it could be a number different reasons for it, bad download or the creator of the rom broke something. This where you would boot into recovery and do a simple nandroid restore. It is nearly impossible to brick a phone these days. It takes a whole lot more then flashing a rom to brick a phone.
So cliff notes:
If phone will not boot up
1. Don't panic
2. Reboot into recovery
3. Do a nandroid restore
4. Enjoy working phone.
This will fix about 99% of all issues with a phone not booting again. If this does not fix it, you are probably trying something else and doing way more then loading up a rom, and feel free to ask for help.
The easiest way to have a always working nandroid restore on your phone is to do a simple back before you flash anything new. This will make sure you always have a working rom with all your SMS, MMS and personal phone settings saved.
I hope this helps a few you out. If not i just killed 10 mins out of a boring day, so I see it as a win, win.