My pc i working fine and evrything but sometimes after using it for a short period of time it starts to slow up a little so i tried scanning for spware,reducing the amount of programs run at the same time,scanning for viruses and many other things until i restarted my computer and it the message "no processor fan detected".I was going to open the computer to try and troubleshoot it myself but i decided to come here first.Help asap

Is it a laptop or pc?

Need more info but opening it up and looking is a good start along with unplugging reconnecting the CPU fan wire on the motherboard. Then try again and hope it was just a tarnished connection.