I'm afraid that I can't answer that, as there is not enough information. Could be anything from a dead battery to a motherboard failure.
All I can suggest is remove the battery for a couple of minutes, then put it back in and see whether it will boot. If it won't turn on at all after that you probably need to get it repaired. If it just goes into the bootloader or recovery you may have a software problem - if so, please post again with as full a description of the state of the phone and what happened as you can.
One other test: if you plug it in to the charger, does the charging light or animation come on (give it a minute to start)? If it doesn't then you definitely have a hardware fault.