Unable to ping to the gateway and all the computers in network

Hi,

i have around 25 pcs in network.from last 2 days one pc in network is unable to ping to the gateway nor any pc.but the icon shows as connected.i tried everything but was unable to solve the issue.i have a router and 2 switches and all the pcs are in same workgroup
. till 31 aug all was fine and from 1 sep its not working.

can u tell me what to do.thanks in advance.