When a computer won't turn on the first thing or things you should check is to make sure the computer has power to it.
If it's a laptop make sure the battery is charged up or at least has some power in it if you're plugging the laptop in.
If it's a desktop make sure the outlet is supplying power and make sure the cord is plugged in to the computer firm enough.
Also make sure the cord is not damaged.
If the computer still does not turn on then it could be a bad power supply or other issue.
Make sure any surge protector or power strip is correctly plugged into the outlet, and that the power switch is on.
Plug in other devices to the power strip and confirm they are operating properly. Double-check that your PC's power supply on/off switch is on.
If you have a desktop computer and plan on trying to repair it yourself, open the computer and verify the power connections.
You can do so by disconnecting the main power cable and reconnecting it to make sure it has not become loose.
Also, verify the power button cable is correctly connected to the motherboard.
Common boot up issues are caused by the following: software that was installed incorrectly, driver corruption, an update that failed, abrupt power outage and the system did not shut down properly.
Let's not forget registry corruption or virus' / malware infections that can completely mess up a computer's boot sequence.