If a desktop computer shows fans starting up but the monitor remains blank without hard drive activity, what is the most likely cause?

A scenario where a desktop computer shows fans starting up but the monitor remains blank without any hard drive activity typically indicates an issue with the system's ability to complete its Power-On Self Test (POST). The most likely cause of this condition is a processor failure.

The processor is crucial for initiating the startup process and performing the necessary checks before sending a signal to the monitor. If the CPU is faulty, the system may power on with fans spinning, but it will fail to progress further, resulting in a blank screen and no signs of hard drive activity since the system cannot run its basic functions without a properly working CPU.

It's important to consider that while power supply failure could lead to a complete system failure, in this case, where the fans are running, the power supply is likely providing enough power to start the fans but may not be supplying stable power to the CPU. Connection issues, while plausible, would generally lead to at least some error beep codes or indications. A corrupted operating system would usually present error messages or allow the POST to complete and start to load the OS. Hence, a processor failure is the most logical explanation for this specific symptom pattern.