Your laptop won't turn on
Is it already on?
The laptop may be on, but it just looks off, e.g. it may have frozen with the screen black. To check for this, hold down the power button for at least 10 seconds, wait for a few seconds and then press it normally again to see if it comes on.
Is the battery flat?
Is it connected to power, and do you know if there is any charge in the battery. If not, then start by plugging it in and leaving it for a few minutes to charge up. It may also be the case that your power adapter has failed. If you happen to have another one nearby that you can try, that would then be worth a shot.
Reset power management chip
If that doesn't work, it's then likely to be the SMC, a chip on the laptop which provides power management. This article explains how to do it