Computer won't turn on: how to tell what you're dealing with

If absolutely nothing happens — no light, no fan noise, nothing — the problem is most often about power actually reaching the machine, not the machine itself failing. Before assuming the worst:

• For a desktop: check that the power cable is fully seated at both ends, and try a different wall outlet or power strip. A tripped surge protector is a common, easy-to-miss cause.
• For a laptop: try a different charger if you have access to one, and check the charging cable itself for visible fraying or damage near the connector — that's the point where laptop chargers usually fail first.
• Hold the power button down for about ten seconds, release, then try turning it on again — this forces a full power drain that resolves a surprising number of "stuck" states on both desktops and laptops.

If none of that changes anything, the likely culprits are the power supply (on a desktop) or the battery and charging circuit (on a laptop) — both of which are real repairs, not user fixes, but knowing it's a power-delivery problem rather than a dead motherboard is useful information going in.

This is a meaningfully different situation than a fully dead machine — it means the computer itself is receiving power and at least partly starting up, but the display isn't showing anything. That can mean the screen or its cable (see our screen display problems guide for that side of it), or it can mean the machine is getting stuck partway through starting up before it ever gets to the point of sending a picture to the screen.

If you have access to an external monitor, plugging it in is a genuinely useful test — if an image shows up on the external screen but not the built-in one, the problem is isolated to the display. If nothing shows up on either, the issue is earlier in the startup process, which usually points toward memory, the storage drive, or the motherboard itself rather than the screen.

A machine that powers on for a few seconds and then shuts off — sometimes repeatedly — is a classic sign of an overheating protection kicking in, or a power supply that can't sustain the load once components start drawing more current during startup. Dust buildup blocking airflow is a common, boring cause of the overheating version of this, especially on a machine that's a few years old and has never been cleaned out.

Some machines are actually trying to tell you something with a specific number of beeps or a blinking light pattern — these are built-in diagnostic codes from the motherboard itself, and the exact meaning varies by manufacturer. If your machine is doing this, the specific pattern (count the beeps or blinks) is worth noting and passing along — it narrows the diagnosis considerably compared to a plain "it won't turn on."

Everything above is safe to check without opening anything up. The next step for a lot of these symptoms — reseating internal memory, testing a power supply directly, opening a laptop chassis to check a battery or charging port — is where DIY attempts most often turn a repairable problem into a more expensive one, especially on laptops where components are tightly packed and connectors are easy to damage. An on-site diagnosis tells you which of the situations above you're actually looking at, and whether it's a quick fix or a bigger repair, before anything gets opened up.