Step 1: Troubleshoot your WiFi adapter:
Are you connecting to a home WiFi router or a public hotspot?
Are other devices or people connecting ok? Did you try connecting a mobile phone to the same WiFi network?
Look to see if you have a network icon in your system tray. What does it tell you?
Does it say no networks are available?
If networks are shown, is the one you were connected to still available?
If yes, then click to connect to that one.
If no network icon visible in tray, go to control panel and then disable and then re-enable the wireless adapter.
Then re-check above steps.
Step #2 Troubleshoot your WiFi router:
If you are using your own WiFi router and nothing can connect to it, you may need to reboot the router (or just turn off the power, wait 30 seconds, then turn it back on).
Give it a few minutes to complete the startup, then repeat Step #1 above.