*Note: This probably isn't your issue.
On October 20, 2014, we switched to using an Autodesk ID for logging into Tinkercad. If your Tinkercad account was created before October 20, 2014, you need to migrate your account to Autodesk ID if you haven't already. Most of the community has already done this, and if you created your account since then there is no need to Migrate.
Important: This is only for people who have a Tinkercad account, but have not logged in since October 20, 2014.
If you've tried everything else and you think you may be having an issue related to this account migration, submit a ticket with the link on the FAQ page or email us at email@example.com.