What should I check first when a computer won't turn on?

Check the power source first — make sure the outlet works (plug in a lamp to test), the power cable is firmly connected at both ends, and the power strip is switched on. For laptops, hold the power button for 15 seconds to drain residual charge, then plug in the charger and try again. If the power light comes on but nothing displays, the issue is likely hardware (RAM, motherboard, or display connection).