If you have removed the battery, you can still run the MacBook by connecting it to an AC outlet with the included AC power adapter.
Unplug the power adapter, Ethernet cable and any other cables from the MacBook. Wait 10 minutes if you have been using the MacBook to allow it to cool off.
Plug one end of the power adapter into the power adapter port on the MacBook and plug the other end of the power adapter into an AC outlet. Turn the MacBook back over and put it on a table.
Press the power button and hold it for up to one second, until you hear the startup tone, to turn the MacBook back on.