TV's can sometimes have difficulty connecting to some wireless networks.
First, try connecting the TV directly to the router using a wired connection to confirm that the TV is indeed working ok.
Then try rebooting your wireless router and try to connect the TV to it again.
If it still won't connect, then you will need to try changing the wireless security settings on the wireless router.
First, try turning off all wireless security - the TV should connect without any problem.
Then turn on different types of security until you find one that is compatible and works with the TV.