Power off the modem, router, and PC. Power up the modem, wait for it to connect and all, power up the router, wait, then the PC. If that did not solve the issue, then do a reset to factory, then you will need to sign on with the default user id and password and re-set up your router entering your WIFI SSID, passwords, ... just like it was a new router.

@ArcherC8 

I already tried but didn't work I also tried to factory reset but I'm not sure if I did it correctly I press reset button with a pen for a few seconds router turned off and turned on but nothing changed router is still on but can't use 

@Lindsey 

Can you sign on to the router? If so, check all of your settings. If not, what lights are on?

@ArcherC8 

No, I can't get in the control panel if you mean that cause I'm not connected its a blue light on the router 

By sign on I am referring to the log in procedure in the user guide https://www.tp-link.com/us/user-guides/TL-WR940N_V6/chapter-3-login#ug-sub-title-1. Some have issues with logging on that way. You can try 192.168.0.1 or 192.168.1.1 instead of the http://tplinkwifi.net/. Sometimes you need to clear history in your browser or even use a different browser. You may need to follow this https://www.tp-link.com/us/user-guides/TL-WR940N_V6/faq

@Lindsey 

I tried to use 192.168.0.1 but it says "Can't reach this page" 192.168.1.1 worked but it's a purple box named Residential
Gateway, it wasn't like that when I was in the control panel.

It appears you have a gateway (modem/router combo). Are you getting any wireless from the gateway? Try disconnecting any connection to the gateway from your PC, connect the router to the PC via Ethernet cable and try using the 192.168.0.1. May need to clear history or use a different browser. Might be time for a new router.

@ArcherC8 

I don't think I have ethernet cable I only have a laptop and phones I never used the cable to connect the internet is there another way to fix it?

The cable between your modem/gateway and your router is an Ethernet cable. One end into the PC and the other into one of the LAN ports. No other cables needed.

@Lindsey 

 

The router will use the IP of 192.168.0.1 or 192.168.1.1. If your modem/router is already using 192.168.1.1 then the router should be 192.168.0.1.

 

Try a different browser and computer if possible. You could also try factory defaulting the router by holding the reset for 15 seconds. When the router boots back up see if you are able to identify the default wireless networks. If you look below the router it should show the name of the wireless network and the wireless password.