I would hold down the power button for about 10 seconds, and then try pressing the power button again.
If you are using a desktop, try unplugging it and then plugging it and plugging it back in again. Then try pressing the power button again. If this does not work, then unplug your computer again, but now plug in something else into the same spot, like a lamp, or a phone charger, and see if they work. If they do not work, then it is the power outlet that is broken, not your computer. If they do work, then there is most likely something wrong with your computer or your power cord. Try powering on your computer using a different power cord. If this does not work, you should take your computer some where to get it checked, and fixed, if necessary.
If you are using a laptop, try charging it, and then hold down the power button for about 10 seconds, and then try pressing the power button again. if this does not work, you should take your computer some where to get it checked, and fixed, if necessary.
