First off, are you able to throw ADB commands to the phone? (a lot of the time when I get stuck in a boot loop I can still throw ADB commands at it), if so, throw adb reboot recovery to reboot into the recovery.
If you can't, remove the battery to turn it off, pop the battery back in, then hold Volume Up, Home, and Power to boot into the recovery.
Once in the recovery do a full wipe.
If you are flashing a ROM it's always suggested to do a full wipe before flashing the ROM.
If wiping doesn't work then restore for a previous backup (I hope you did a backup before you did whatever you did since you weren't being detailed in what you were doing at the time).