PC won't turn on? Diagnose the symptom before buying parts.

Stop first if there is smoke, liquid, heat, or a burning smell

Turn the power supply off, unplug the computer, and do not keep testing it. Never open a power-supply enclosure; it can retain dangerous energy even when disconnected. After liquid exposure or visible electrical damage, use a qualified repair service instead of repeatedly applying power.

Branch 1: absolutely no power

1. Prove the outlet. Plug in a simple known-working device. If you use a surge protector or UPS, confirm it is switched on and has not tripped.
2. Check the external power path. Reseat the power cable at the wall and power supply. Put the rear power-supply switch in the I position.
3. Discharge and retry once. Switch the power supply off, unplug it, hold the case power button for 15 seconds, reconnect, and try again.
4. Inspect the main connections with power removed. The motherboard normally needs a 24-pin cable plus a separate CPU power cable near the top edge. Do not substitute a PCIe/GPU cable for the CPU cable.
5. Check the case-button connection. Compare the tiny front-panel connector position with the exact motherboard manual. A loose power-switch lead can make a healthy computer appear dead.

If this started immediately after a build or part change, reverse only the last change and retest. Do not open the power supply or bridge unfamiliar pins.

Branch 2: fans or lights, but no picture

1. Verify the monitor. Confirm power, correct input, cable seating, and a known-working cable or display when available.
2. Use the correct video output. With a dedicated graphics card, connect the monitor to the card. Motherboard video outputs require a processor with supported integrated graphics.
3. Read the board before moving parts. Diagnostic LEDs commonly identify CPU, memory, graphics, or boot-device stages. Use the manual for your exact model; the light is a starting point, not a guaranteed failed-part verdict.
4. Power down before reseating. Unplug the system, discharge static by touching grounded metal, then reseat memory. Test one module at a time in the manual's recommended slot.
5. Check the graphics path. Reseat the card only if you are comfortable doing so, confirm its power connectors, and verify that the card is fully latched and physically supported.
6. Allow memory training time. A new build or major firmware change may restart or sit on a blank screen while memory is trained. Follow the board maker's documented timing before interrupting it.

Branch 3: the PC shows a logo but Windows does not finish starting

1. Remove new external devices. Disconnect nonessential USB storage, hubs, and accessories, leaving only display, keyboard, and mouse.
2. Record the exact behavior. Photograph the error, note whether it loops, and write down the last update, driver, program, or hardware change.
3. Use Windows Recovery Environment. Microsoft provides Startup Repair for common startup failures and Startup Settings for Safe Mode and other diagnostic options.
4. Protect encrypted data. Recovery may require the BitLocker recovery key. Do not reset, reinstall, format, or erase a drive until important files and recovery credentials are accounted for.
5. Change one variable at a time. If Safe Mode works, investigate recent drivers, startup software, and updates before blaming hardware.

What the evidence means

• No response at all: focus on the external power path, switch, case button, and motherboard power—not Windows.
• Diagnostic memory light: verify slot choice, seating, one-module testing, firmware compatibility, and the board manual before replacing RAM.
• Picture appears, then Windows fails: the display path and much of POST are working; move to recovery and software evidence.
• Problem began after one change: undo that change first. A timeline is often more useful than a pile of replacement parts.
• Intermittent shutdown under load: record temperatures, power connections, event timing, and workload. Do not assume wattage alone proves a power supply is healthy.