Hi Francesca L,
Thank you for reaching out. My name is Leo, a user just like you. I'd be happy to help you with your concern.
May I know which account cannot be access? Is it a work or school account or a personal account? May I also know if you can log in via Teams online at www.teams.com using a browser? Can you log in normally without seeing an error code or message? If you can't log in online, there may be an issue with the account. If it's a work or school account, these accounts are managed by an organization. To do this, you will need to contact your company or school's IT administrator to help you with this issue. If you can reach them, you can inform them of the problem and ask them to remove your account access and add it back. After that, try logging into the account and see if you can log in afterwards. You can also try uninstalling and reinstalling Teams via Microsoft Store.
Your understanding and patience will be highly appreciated. I hope I provided a helpful solution regarding the issue! Let me know if you have any further questions. Be safe always!
Best Regards,
Leo