Welcome to Android Central! Can you tell us what kind of phone, which network, and a little more detail about what you did?
--What do you mean when you say you restarted your phone then you turned it off, now it won't turn back on?
--Did you power the phone down normally by holding the power button down, then selecting power off?
--Did you use any other kind of reboot procedure, or pull the battery?
--Was the phone acting funny beforehand?
--Was your battery close to empty when you tried to restart, and have you tried plugging the phone in overnight?
--Have you tried holding the power button down for a good 10-15 seconds to see if it will power on?