Sometimes, when you try to connect your Chromecast to a PC running under Window OS a sign may appears telling that the WiFi connection is off, although it is working correctly.
So, how to use Chromecast on a Windows computer correctly
To send and cast streamed media from a Windows PC to your TV via a Chromecast dongle you must have the most recent version of the Google Chrome browser, one or more applications that support Chromecast, such as latest version of the BBC iPlayer, Netflix and so on.
If you want to cast any media content stored on your PC you need two Chrome browser add-ons called Chrome Cast and Videocast. They can be downloaded in Chrome by going to Controls > Tools > Extensions. Providing your setup meets all of these criteria, and the PC is connecting normally to the Internet then it should work. Try running the initial setup again, but first, perform a Full Data Reset (FDR), which puts Chromecast back to factory condition. To do that hold down the button on the dongle for at least 25 seconds while it is plugged in until the light starts flashing. It may also help to clear the router’s cache memory before you begin, so switch it off, and anything that connects to it. Then switch the router back on and wait for the lights to stop blinking before powering up the PC, then plug in the Chromecast.
If the problem persists there may be some configuration issues with the router Also the VPN service you may be using can stop it working and if all else fails there is a simple to follow troubleshooter on the Chromecast FAQ.