Confluence will fail to start if the build number in your database does not match the build number of the version of Confluence you are attempting to start. This is to prevent data corruption. 

This can happen if:

• You have started a newer version of Confluence in your test, staging or development environment, but didn't change the database configuration, causing your production database to be accidentally upgraded. 
• You are attempting to downgrade Confluence by running an earlier version of Confluence against your existing database.  It is not possible to downgrade Confluence. 

To resolve this issue you will need to: 

1. Head to Confluence build information to look up the Confluence version associated with the build number in your database.
2. Upgrade your Confluence instance to this or a later version.