Looks like either someone's been messing with the metadata or there's been some odd problem dropping an index somewhere. The index structure still exists but the metadata's gone.
Recreating the table might help (select into, drop the old table, rename the new one, recreate all indexes). If it doesn't, you'll need to recreate the database.
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