Error attaching database after deleting NDF file
Based on all of the information you've provided so far, I'd have to say that, yes, you've lost all of your data. From what it sounds like, I can only guess that something like this happened:


your disk filled up
you saw this big .ndf file
you tried to delete it (presumably because you didn't realize it was a secondary data file)
you couldn't delete it, because it was in use
you stopped SQL Server (or detached the database)
now you could delete the file - yay?


The problem is, the .ndf file is a crucial part of your database. The fact that you couldn't have possibly deleted it without detaching the database or shutting down SQL Server should have been a clue.

Have you not taken any backups of this database since adding the .ndf file? Preferably from as close to the point that you deleted the file as possible?

Now, if you don't have a database backup (or a simple file copy of the .ndf and all the other files in a consistent state, after a clean detach), it is possible that not ALL hope is lost. You may be able to recover the file using some kind of undelete software or taking it to a data recovery service. Provided that the location on disk where the file used to live has not been zeroed out, formatted or re-occupied. You might get lucky, but odds are what you've done here is essentially the same as dropping a donut in the toilet. You might be able to get part of it back, but you still might not want it.

Unfortunately this is called learning why backups are important, the hard way. And also serves as a good warning to others that they shouldn't be deleting files on their system if they don't know what they are for.