No screen (PC on, fans spinning)

The PC is turned on, fans are spinning but the screen remains black / no video signal is received.

Multiple causes are possible; some things that could cause this based on previous experiences:

• Have you checked the connection between the pc and the monitor? Really? Both sides of the cable?
• If a video card is connected, main video output defaults to the card instead of the CPU/motherboard internal graphic. Check that the cable is connected to the video card.
• Video card could be faulty, disconnect it (after powering down the pc) and test with the CPU/Motherboard internal graphic if present, otherwise test with a known-functioning video card (check that it is in fact known-functioning in the Tarallo).
• Check if the video card requires a dedicated power connection (if the power supply doesn't provide a PCI power cable, swap the video card or the power supply).
• Check if the CPU requires a dedicated power connection (4/8 pin connector).
• Check the list of CPUs compatible with the motherboard.
• Check that the RAM is not ECC (if the system doesn't support it).
• Make sure to test the power supply with the tester (article here). A faulty power supply (usually with some blown filter capacitors) can start the machine but won't even allow it to boot.
• Visually inspect the board for bulging capacitors; just one capacitor in a strategic position can prevent the board from booting. Sometimes the defect is not obvious, it's worth searching the web with the board model + "blown capacitors". Once the problem is identified, replace the capacitor(s).
• If the PC is marked as working but has been idle for a long time, try a CMOS Reset and remove and reinsert the RAM, making sure to remove dust from the slots with compressed air.
• If none of the above options proves correct, solve the problem and then write it in this article to make life easier for future WEEEinians.

Seriously speaking, the CPU could be gone (but this generally stems from other related causes), the northbridge could be gone (irreparable for us) or there could be a thousand other problems.

Before throwing the PC in the bin, always consult with another WEEEinian or two, rethink all the troubleshooting you've done, then think about it again, and if the motherboard really needs to be thrown away, recover everything recoverable (Case, CPU, RAM, disks, jumpers, standoffs, optical drives...) before putting it in the WEEE bin.