Multiple mdf, empty one is missing
Might sound silly but first thing, check your recycle bin. If the secondary file was small/empty it may still be there.

IF it is, shut off sql server, go to recycle bin, right click the file and choose restore. Then restart sql server and see if it works. 

IF you did not drop the secondary file from the filegroup then the database expects there to be more files, so the error "inaccessible files" makes sense.