We have a user that is trying to access a site collection. I've given her owner rights but she is still unable to access the site collection. I think it's worth mentioning that this user left the organization for awhile and came back. It almost appears as though her profile isn't syncing properly. Is there a way to force a resync with Active Directory? We aren't currently using the User Profile Sync service to my knowledge.
If the user's AD account as deactivated, and then reactivated, then they should still have access. If their account was deleted and then recreated, then they have a new SID and are not the same user.
I think the Move-SPUser PowerShell cmdlet will help you fix this. (Do a web search first.)