You can reuse the physical datafile but you can not salvage the
contents. The alter database drop datafile action is a permanent
action.

If you are trying to add the file back to the same tablespace you will
need to drop the entire tablespace including contents prior to adding
the file to the tablespace.

alter tablespace
add datafile 'xxxxx' size 1024M reuse;

But this will reformat the file so the contents are toast.

You should not issue any command to alter the physical database until
you have researched its effects.

HTH -- Mark D Powell --