There are a handful of activities that have broken my SQL Server backup routines. Sometimes a DIFFERENTIAL (or LOG) backup would fail because there was no FULL backup. Other times, a LOG backup would fail because the log chain was broken. Some of those activities include:
Creating a new database.
Changing the recovery model to SIMPLE.
Restoring/reverting a database from a snapshot.
Any other activity that breaks the transaction log chain.
Dave discusses an easy way of figuring out if you’ve just performed an activity which breaks the log chain.