Start with the right checks.
A startup error is different from a mailbox that will not sign in. Keep the exact error and establish whether you use classic Outlook or the newer Outlook application.
What to do next.
Save other work and restart the PC normally if it is safe.
Check whether your mailbox opens through its official web service. That comparison can separate account access from a desktop problem.
For classic Outlook, use Microsoft’s startup troubleshooter in Get Help on the affected PC.
Keep the result and any recent add in or update changes for the person assisting you.
Check the result.
Open the intended mailbox and check recent messages, calendar and any local folders. A new empty profile is not proof the original information is safe.
Stop and get help if…
A repair would remove a profile, a local archive is involved or the same error returns. Ask for a plan that protects the existing mail before more changes.