"My PC won't turn on" is a phrase that has many meanings, so it's important to be able to specifically check what exactly isn't turning on and how to check to see if there's an explanation for why it's happening.
Below are a handful of scenarios of a PC not turning on, but with something that we can look into.
My PC doesn't turn on at all. No lights, no fan movement, nothing.
Double-check that every connection on the motherboard is fully connected and plugged into the motherboard.
It's also worth doing tests on the Power Supply (PSU) to see if the PSU has failed. Learn more on how to do that here: How to test a PSU (Power Supply Unit)
The PC turns on, the fans spin, but nothing shows up on the monitor.
If your system does turn on but is not outputting any video signal, double-check all cable connections from your monitor to your system. Make sure they are not loose and that they are all properly connected.
For systems that have a dedicated graphics card, make sure you're connecting the display cables directly to the card and not connecting to the onboard video outputs on the system motherboard (which will be higher up on the system when looking from the back).
Make sure that you have selected the proper input on your monitor. Most monitors have the feature of selecting the input that the monitor receives a signal from, so changing between input sources may help.
Debug Lights
Depending on the motherboard, your motherboard might have a row or column of LEDS (or a Seven-segment display that has a line of LEDs underneath) in the top right corner of the motherboard.
Those are the Debug Lights and exist to give you awareness of what's being done. The typical order for it is:
CPU> RAM > GPU > Boot Device.
If a light stays on for a long time, that can indicate a problem with that particular component.
Reseat Components
Try reseating the RAM, and test one module at a time in different slots.
Try reseating the GPU. Learn how to do that here: How To: Reseat A GPU/Video Card/Graphics card
Try reseating the CMOS Lithium battery, which might be located underneath the graphics card or near the CPU.
Laptop?
For notebooks, remove the battery and power cable, hold the power button for 10 seconds then wait an hour. Reconnect the battery and power then try powering it on again. Try the display mode shortcut keys (FN+F# key, usually).
If you need further assistance, please contact our Support team.