Does it work again if you clear the cookies?
Also, it's just a thought, but I'd shut all devices connected to the modem, whether by WiFi or Ethernet, then power cycle the modem, meaning unplug it, wait for at least a minute, then plug it back in. After waiting a couple of minutes for it to be fully back up and ready, try it again. This may help if it's a DNS related issue.
You could also temporarily try using a public DNS service, like Google's or Cloudflare's. There's a way you can change the DNS in the modem's settings. It's in the Advanced Setup section, the same section where you can adjust the WiFi settings. You can uncheck Obtain from ISP and add the DNS settings, then click Save Settings. I don't know that this will make any difference, but it's a fairly easy thing to try.