Hi Nicole,
#1 Go to https://admin.microsoft.com/#/homepage
#2 Type in the user in the search at the top:
#3 Choose the user and a dialogue window will open > hit Unblock sign-in
You can also do this from Azure Active Directory:
#1 Go to https://aad.portal.azure.com/
#2 Go to Users
#3 Search for the user and click on their name.
#4 Go to Edit under Account status
#5 Click the checkbox for Account enabled > Click Save
If this is helpful please accept answer.