I used to open sdf (sqlCE) files with visual-studio? or sql-server? I really don't remember.
Now I can't open this sdf file. With what program do I need to open it?
It's a SQL Compact database. You need to define what you mean by "Open". You can open it via code with the SqlCeConnection so you can write your own tool/app to access it.
Visual Studio can also open the files directly if was created with the right version of SQL Compact.
There are also some third-party tools for manipulating them.