This was the fix when TEAMS had stopped working/became unavailable in the USER 365 login. It also fixed a related issue with Outlook missing from and individual user's 365 account.
1) Login to the https://admin.exchange.microsoft.com/
2)) Login as Genesis Admin (mailto:genesis@domain.onmicrosoft.com)
3) Navigate to 365 Admin Center--> Users-->Active Users-->
4) Select User --> select "Licenses and apps" from Menu --> Scroll to the bottom
5) Select "Apps"
6) Be sure every relevant app/service is selected