You do not attach datafiles to a database.

You are cloning -cloning as in creating- a database based on "all necessary files, including control files, redo logfiles, datafiles" you have gotten from other one.
Thanks PAVB.
I have just got the files of a database for which instance files were 'deleted', with rm command. From what I read about cloning is ,database needs to be cleanly shut down to perform a cold cloning. I have installed Oracle software with the same sid, port and home directories, but when I try to connect as sys, it gives the error that I've already posted.
Any help/suggestion is appreciated.

Thanks
gtcol