Final answer:
The likely reason for case fans spinning but no power to other devices could be improper connectivity, where cables are not correctly attached, or there is a motherboard issue preventing proper power distribution.
Step-by-step explanation:
A possible reason for the symptom where the power supply fan and case fans spin but there is no power to other devices could be B. Improper connectivity. This can occur when the cables connecting the power supply to the motherboard, or other essential components, are not properly attached. It is also possible that the motherboard has suffered damage or has developed a fault that prevents it from distributing power correctly. Signs of this would be fans spinning up because they are directly powered by the PSU, but other components like the CPU, graphics card, and RAM not receiving power because the motherboard is not functioning correctly.
Other possible reasons can include a faulty power supply that does not provide adequate power to all components or a problem with the PSU connectors themselves. However, failed hard drives, drives not being recognized, or failed RAM would typically not prevent power from reaching the motherboard or other devices; instead, these issues would result in boot errors or system instability after power has been distributed.