Your question doesn't seem to be associated with programming. Yet you posted in the programming forum..
How are we supposed to help you if we don't know what your project is or what you have hooked up to the Arduino?
Since your question is so vague, I will give a vague answer; the most common reason why the Arduino restarts is you're pulling too much current.
Also, the arduino could reboot if you connect the 5v to ground with no resistor.
It's not likely, but you could have connected the reset pin to ground, which also resets the board.
Shouldn't this be in electronics?