Sending, receiving, and organizing email in Outlook.com
Thanks for reaching out. Before anything else, did you try signing in with a different Microsoft account on the same devices to confirm if the issue only affects your main account?
When To Do stays stuck on the loading screen across all platforms, Microsoft explains that it usually points to a service issue linked to the account itself. The best step is to check your Microsoft account status, confirm your Outlook storage is not full, then remove and readd the To Do account on each device. If the issue happens only on your primary account, you need Microsoft Support to review and repair your To Do service data on their side, since only they can fix account-level sync problems.
Here's the link: https://support.microsoft.com/en-us