Can a database file be brought back online if the file is recovered from a disk backup?
I'm 99% sure that setting a file to offline is a one-way operation. The only route to bring them back online is to restore the data from a full backup and then apply log backups.

There may be an obscure hack route to get this working but I'd be inclined to rebuild the database. Create a new database, transfer the schema you want to keep, copy the data across.