Hello Claire
Thanks for your question
If they are Azure accounts, you will need to contact the global admin in that tenant help prompt a reset of your MFA methods if you do not have access to the app again if it is a work/school accountin Microsoft. For other apps you will have to contact their admins
If unfortunately you can't do the above, the only resolve this by contacting support or asking your admins/support to reset your account as mentioned here https://learn.microsoft.com/en-us/xandr/curate/troubleshooting-multi-factor-authentication-mfa?source=recommendations
To contact support you can call the Global Support lines here: https://support.microsoft.com/en-us/topic/customer-service-phone-numbers-c0389ade-5640-e588-8b0e-28de8afeb3f2
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola