IMac will not connect to HP Officejet Pro 8620.

I just bought a HP Officejet Pro 8620 printer. Initially, the wireless setup went great. Everything worked. Next day everything stopped working. Now I have gone thru all the steps in trying to connect, but failed. I get a message "device not reachable" but my IMAC, printer and router are all next to one another. Makes no sense to me. My IMAC is wireless and connected to the router and my printer is connected to the router. But the IMAC does not see the printer. Can someone help me?

Thanks