Hi,
It could be a faulty fan or as I said it's not passing POST.
If the CPU fan is not working then most likely the CPU will be shutdown to prevent any damage occurring to it due to overheating.
Most CPU fans have an rpm sensor and also a temp sensor embedded in them so that the BIOS knows what speed the fan is operating at (which also lets it know that it is working) plus the temp of the air being drawn over the CPU so it know how hot it is.
Being sometimes intermittent the quickest way to know what's happening is to connect a system speaker (known as a chassis speaker in the [https://download.asrock.com/Manual/B450M%20Steel%20Legend.pdf|user manual] for your motherboard) to the motherboard and check for any beep error codes when the PC is first started - see images below for connection location and code meaning
[image|2811785]
[image|2811786]
(click on images)