[Zero Two]

A PC that won't boot is one of the most common and most solvable problems, but only if you diagnose it systematically instead of randomly swapping parts.

What does "won't boot" actually mean?
This matters. Different symptoms point to different causes:

• No power at all (no lights, no fan spin): power supply, power button header, outlet
  
• Powers on but no display: GPU, RAM, CPU, monitor connection
  
• POST beeps / LED error codes: check your motherboard manual, these are diagnostic codes
  
• Gets to BIOS but won't boot OS: storage, boot order, OS corruption
  
• Gets past POST, BSOD/freezes during OS load: RAM, drivers, OS corruption
  

The systematic approach:

1. Breadboard test - strip the system to minimum: CPU, one stick of RAM in slot 1, GPU (or onboard if available), power. Does it POST?

2. Check RAM - try one stick at a time in the correct slot (A2 on most boards). RAM is the most common cause of no-POST.

3. Check GPU - if you have onboard graphics, remove the GPU and test with that. If no onboard: reseat the GPU, try a different PCIe slot, borrow a known-good GPU.

4. Check storage - if it POSTs but won't boot OS, try another storage device or boot from USB to rule out drive failure.

5. Check power supply - PSU failures are common and PSU testers are cheap (£10). A failing PSU can cause all kinds of intermittent symptoms.

Before replacing anything:

• Reseat every component (RAM, GPU, CPU if you're confident)
  
• Clear CMOS (there's a jumper or button on the motherboard)
  
• Check for bent CPU pins if you've recently handled the CPU
  

Post your specific symptoms and I'll help narrow it down.