Hi,
Does this issue happen suddenly? If so, please try the method below.
Open Internet Explorer by clicking the Start button . In the search box, type Internet Explorer, and then, in the list of results, click Internet Explorer.
2. Click the Tools button, and then click Internet options.
3. Click the Security tab.
4. Now click on Trusted sites and then click on the sites button.
5. Enter the Website home.dynamics.com and click on Add.
6. Additionally add if not added
 *.crm*.dynamics.com
 login.microsoftonline.com
 portal.office.com
7. When you finish, click on Close.
8. Try to login to Dynamics 365 again.
Hope it works.
BR,
Judy
