I am facing a similar problem like Helen. My database requires a media file recovery. But it's even funnier as this one. As you replied you need to connect as sysdba and then recover the datafile. This doesn't work for me. I'm not able to connect as any user within the database. I always get the message initialisation or shutdown in progress.
Is there another approach for recovering this datafile? Or how can I detect wheter it's already in progress recovering the datafile?
Thank you very much for the tip of checking the password file. I had the idea it has nothing to do with it. But when i changed the parameter to shared and resetted the password again. I managed to get the database up again.