Linksys Cisco Router help?

ok so here's the problem, at my house we have a wireless router that is linksys cisco router (http://www.bestbuy.com/site/Cisco+Linksys+-+Factory-Refurbished+E1000+Wireless-N+Router/1401676.p?id=1218255557761&skuId=1401676) and we have three computers in the house that can connect to it. but there is one computer that wont connect you can use the internet but only when plugging in an ethernet cable. so whats the deal why wont this 4th computer connect to the router?