Recently one of my pfSense boxes wasn't handing out IP's. In the DHCP log it said: "There's already a DHCP server running".
Here's how to fix it:
Go to Diagnostics -> Command Prompt and execute this shell command:rm /var/dhcpd/var/run/dhcpd.pid
Make sure to restart the DHCP server once this is done (in Status -> Services) (thanks)