My laptop can't see the printer even though both are connected to the network

[#1]

For some reason none of the laptops on our home network can find the printer even though it is connected to the router. I don't think we have changed anything - it was working fine until today. I've re-entered the router settings into the printer, switched it off/on, spend 1hour with apple support trying to see if it was a problem with the laptop (but same problem with windows laptop too); restarted the laptops, disabled Sophos, checked it is 'shared' on the desktop computer (which the printer is connected to via usb) ..................Logging in to the router shows the printer & laptops active and the wifi light is definitely on on the printer. The only way I have managed to connect is by plugging the printer in via a USB cable. What else can I try?

[#2]

I take it you have had it working with both wifi & usb simultaneously before?
As a test unplug it from usb.
Change the wifi settings to DHCP.
Check the ip address it is given on the printer & router match.
You might have to reset up the driver on each l/t if the ip address has changed, try a search and see if it detects the printer.

[#3]

Just tried unplugging the printer from the desk top computer and miraculously my laptop could 'see' it - and I've managed to print from the laptop. Plugged it back into the desktop -  and it still works!
My son's laptop could also see it when it was unplugged but as soon as we plugged it back in to the desktop, it appeared on his laptop to be offline again. He can only access it if I share it from my laptop. (It is set to 'share' from the desktop too.)
The ip addresses match and yes, it has worked for months with the usb for the desktop and on wifi for the laptops.
Not sure how we change the DHCP wifi settings?  Could not find that on the printer setup.
Many thanks for all your help.

[#4]

Just as I posted, my son's laptop showed the printer as 'on line' again. We didn't change any other settings so not sure what happened.
My son wondered if it is a problem with our home network - don't think I am 'techie' enough to sort that out so let's hope that it keeps working now we have it sorted.
Thank you Hairy McBiker for your time and help. 

[#5]

Is it possible that a domestic appliance or Central heating had interfered with the WiFi connection, for the duration of downtime
Also check your WiFi channel for interference / crossover by neighbours
Channel Numbers 1, 6, 9, 11 are the most popular default by ISP's & can get quite congested in highly populated areas
You can use a programme like Inssider for Windows / Mac or Android to check
If you need any help to change wireless channels then just hollar back & we'll be glad to help

[#6]

I'm interested to know why you are using USB and wifi on the same printer?