A file should only be set offline when the file is corrupted, but it can be restored. A file set to offline can only be set online by restoring the file from backup. For more information about restoring a single file, see RESTORE (Transact-SQL).

You will need to restore the file from backup. If you do not have a backup, according to the article, you will not be able to get the file back online.

http://msdn.microsoft.com/en-us/library/ms190241.aspx