iMac won't turn on and stays black

I am still using my iMac, but then my eoectricity in the office room got shut down autmatically and unplugged my iMac. After replaying the cable in the office in, I turned on my Mac and logged in successfully, but then my YouTube doesn't respond, and I tapped my whole hand on the iMac gently in the he monitor, and then it turned off, and I cannot power it on. Even if the electricity is still on and I can still charge my iPhone at the same plug.