If you have internet, then check if your server is still running correctly.
If you are using Chrome, try this:
Chrome settings -> Privacy & security -> Site settings -> Cookies & site data -> Allow it if blocked.
On any browser, disable ad- and cookie blockeras on that page and see if that helps.
Mostly this error is caused by cookie issues.
You also could try another browser and see if it works then, that would confirm it's a browser cookie or cache issue.