It happened to me earlier and I've seen a few people in the same boat.
So you flashed a zip, rebooted and stuck at splash. When rebooting to recovery it goes and sticks on splash and the only thing to work is fastboot.....what to do?
Don't panic its not a brick.
Hold power 7 seconds to restart phone, when off hold vol down to enter HBOOT.
Once in HBOOT press power to enter fastboot.
First try 'fastboot erase cache' and then 'fastboot reboot-bootloader' this should do the job, if this don't work, reboot into fastboot and try below.
On your pc open cmd and change directory to where your adb and fastboot are. Once there use 'fastboot oem lock' your bootloader should now say Relocked I think.
After that download RUU specific to your handset and run. This should install fine and return you to stock.
After done you may need unlock bootloader again, I did via HTCDEV and then reflashed recovery with 'fastboot flash recovery *recovery name*
Job done, phone restored, panic over :)
Sent from my HTC One X using XDA