It would be useful to know the context in which this error occurs:
- When running certain queries
- When using management tools
- When buying a ham sandwich
I would guess that it means that the operation giving the error is trying to join two table on columns with different collation settings. Why they now have different collations I don't know - have databases been moved between servers or some such
One particular thing to watch out for is the server level default collation as this is (I believe) inherited by tempdb. You need to check the collation settings of the server, and the particular database, tables, and columns causing the problem.