SharePoint o365/online - This site can't be reached

To troubleshoot the issue, check things below:

1. Check if the URL of the SharePoint site is correct.

2. Check if the licenses in O365 are expired.

3. Clear the cache in IE.

4. Add the SharePoint online site into the trusted site in IE.

And you also could follow the troubleshooting steps in the article below.

Fixing Microsoft Site can't be reached error in Chrome.

If the issue still exists and you can access the Microsoft 365 admin center, then you can submit a service request like the picture below.