This is normally caused by blocked or lost traffic, this happens way more often on Public networks like McDonalds wifi or College dorm wifi. If this is happening on a private home network, there is normally a more "fixable" cause.
Firstly, restart your router or modem, always a good start.
To make sure it's not a local issue with your pc, you can reset and renew your IPConfig, and Flush your DNS. Restarting your PC wouldn't hurt too. Be also to make sure nothing is being blocked in your firewall.
If none of the things helped, it's also possible it's a temporary issue with your ISP, sometimes these things happen. But if it's been an issue for long, it's more likely that it isn't that.