Generally SQL Server is not Case Sensitive; however, it is possible to set up the Collation
of databases, tables, and the respective content which will make SQL Server case-sensitive.
If everything was working fine one day and then stopped; the first thing I would be checking is to see what changed. I think this would qualify and I would be checking what was changed at the database; and not in the client code. I have a strong feeling that there was some sort of collation change.
Perhaps a read through the problem, answers, and referenced articles here will be of assistance:
Collation: SQL Server Column names case sensitivity - Stack Overflow