I am experiencing this error message on my SQL Server:
Invalid database file version
The .MDF in question was working previously.
How can I fix this because the information in the file is very important to me?
It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, see the FAQ.
Without the actual error message I'm just guessing here...
Odds are the database you are attempting to attach is newer than the version of SQL Server that you are trying to attach it to.
For example you have a SQL Server 2005 instance, but the database file was last attached to a SQL Server 2008 instance.