If your dsl modem/router is plugged into a power strip, try changing it out. I've seen faulty power strips cause quirky behavior in the field when related to modems, routers, switches, and other network equipment.
When you're unable to see the printer, check to see if you can still surf the web. Open up the command prompt and enter 'ping google.com'.
If you can connect to the internet, see if the printer is sleeping. Wake it up and see if it reappears in your Network.
The printer should also have an IP address. In Windows 7 you can find it by going to Start->Control Panel->Hardware and Sound->Devices and Printers. Right click on your device and then click on 'Printer Properties'. Click on the Ports tab and scroll down and find the one that is checked. This should give you the IP address (ex: 192.168.1.113) <-- the number will vary depending on the router setup. Like before when checking on the internet, open up a command prompt window and enter 'ping 192.168.1.113' <-- replacing the IP address with that of your printer. If you get a response back from the printer, but cannot see it or print to it, you might try reinstalling the print drivers for your device.