If it has set as access point the biggest problem may be that you lose dhcp and all ip info. With the router on carefully press the reset for about 5 secs. As you release it the lights should flash and it will reboot. The default setting for it's ip should be available. See the setup instructions for this. Go to your wired network connection properties and set the IP address to the same first 3 numbers, then set the last to a few numbers away from this.
(If the router default is :
192.168.1 254 set 192.168.1.250, you can not use 255 or higher. for router default :
192.168.2.1 set 192.168.2.10)
Connect to the router with the LAN cable supplied. In your browser enter the router address, say 192.168.2.1 and enter.
The admin password is normally admin or is left blank. Check the setup instructions. Go to the setting for type and ensure router is set as a router. Then carry out the full configuration, saving each change as you go. Test the final config with the cable, set up encryption, I advise changing the admin password to something you will remember. Remove the cable. Then use the wireless utility on any machine you want to connect, search for your network set the encryption and connect. Return to you wired card tcp/ip and re-set it to auto.