when macbook pro sleep don't start until plugged

I have one macbook pro 13-inch early 2011, and when I close the monitor or put then to sleep , he turn off the screen and light on the front flashes, so if I open the monitor the computer wakeup but if the light stop flash I only can turn on my computer if I plug the power cabe, even if I press power button (with power cabe disconnected) don't works

anyone know why??

tês