If the PC isn't seeing the router at all, and it's wired check the wiring
The fastest way to check is to look at the lights on the router or the back of the PC where the cable goes in, normally at each end (router and PC) there will be indicators to show if there is a basic electrical connection
IE your router might have a pair of LED's per physical port, one to show it recognises a cable being connected to it and another device, and a second one for activity.
If the connection lights are not lit, it means generally either the cable is unplugged (or making a bad connection) at one end, push them in and see if the light turns on, or the cable is damaged..
The other possibility is that the either the routers LAN port has failed, or the PC's network card is disabled or has failed.
Basically, if it's a wired connection to the main PC, try unplugging the cable and plugging it back in at both ends, then try a different port on the router, and a different cable (or try plugging your mac book into the cable instead of the PC and see if it connects via the cable).
A spare network cable is always handy when having network connection problems
(I tend to take a spare cable and an old router when I go to fix friends internet connections, as it lets me rule out the cabling to the PC and the PC itself very quickly)
