Either the database was switched to simple recovery or the log was truncated explicitly (which isn't possible on SQL 2008) or the database was reverted to a snapshot. That's about the only things that will break the log chain and have SQL know about it.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability