Try other browsers to verify it's not just confined to a browser issue?

Does Google redirect to the sign-in screen? What address is Gmail stuck on?

If you are using an old bookmark, try

https://mail.google.com/mail/

My gmail does not open when I click on it. I just get a blank page.

Show us what you're getting. It works for me. see screenshot

Cannot open gmail with firefox. Only opens when I switch to troubleshoot mode. Works in other browsers.

One of the main features of Troubleshoot Mode is to disable your add-ons. I don't know whether you have any extensions that might block content (ads or other content) or restrict cookies, but it would be worth testing whether it helps to make an exception for Google sites.