Check that the router can see your computer by checking on the Device List. If your machine is there, the connection is intact. Ensure that the wireless connection is enabled on the client machine and that any firewall is allowing the connection through. On a Windows machine, go to Control Panel, Network and Internet, Change Adapter Settings, right click, select Properties, Configure then scroll to the Channel setting.
Ensure the wireless channel on the client machine matches the one configured on the router. To be extra sure, change them both to something different and save the settings. If this makes no difference, very briefly turn off all security on the wireless connection and try to connect.
If you can connect, reset the security, and the username and/or password. If it makes no difference, switch it back on again. Next check the type of wireless connection you have configured. Making sure whatever settings you have on the router are mirrored on the client PC. Especially things like security settings, channel, security type, connection type, 802.11b and so on.
As long as all the settings mirror each other on the router and client, the connection should work.
As a last resort, at the back of the router is a small hole, which is the reset button. Pushing a pen or pencil into the hole and holding it for a couple of seconds will reset the router to its default settings. This will give you a baseline to work from, but means you will have to configure it again. This can often resolve issues, as all the settings are entered again, and any configuration issues are addressed with the new commands.
If that doesn’t work either, refer to the Linksys site, and go to the Support section. Follow the troubleshooter I linked to above, or find the latest firmware for the router model. If yours is older, update it.