I'm on application is known as SQL Server 2005. I have been severely working with it during some years. Once bad thing happened – I made an effort of urgently opening .mdf and unfortunately, I couldn’t, SQL Server viewed could next: even neither open nor gives messages.
do you have a backup of this precious database? 
.mdf is the default extension for a database file. It is not opened directly, rather it is opened by SQL Server. Are you trying to attach it to an instance? How are you trying to "open it"?