That would depend on what color light it is flashing. If it is flashing the orange light, then it's because the power bar is overheating. Your power bar is sort of going into safe mode, and will probably not let the Xbox turn back on until it has cooled down. To prevent this from happening, try not to leave it running all day, make sure there's at least 24 inches of space on all sides of the 360, never put anything on top of the console, and never unplug the console while it's powered on.
My boys Xbox 360 has been sent back three time and I'm getting ready to send it back a fourth! It's four years old! Their's has "the red ring of death" meaning it to hot or ready to crash, us it depends on how many lights are flashing!