How long did you wait - it can take a few minutes to boot up the first time due to rebuilding the cache and running initialisation scripts? If it's definitely hung, pull the battery, wait 5mins or so, replace and reboot.
If it still hangs then pull the battery again, but this time boot into recovery by holding HOME when powering up. Choose "install zip from sdcard", "choose zip from sdcard" then navigate to the /clockworkmod/download folder. Somewhere in there will be the ZIP flashed by ROM Manager. You should wipe the cache and dalvik-cache partitions before reflashing, but you might be advised to do a full wipe to clean up any corruption from the failed attempt.