Unable to open database (Datafile corrupted)
Ok, so you have no backup.

You made sure it is not a permission issue.

But you have Data Pump dumps, at least that is something.

First try to recover the datafile without data loss:

startup mount
alter database datafile 'V:\DB\CST001.DBF' online;
recover datafile 'V:\DB\CST001.DBF';


Answer the prompts accordingly, if any. If this succeeds, you can open the database with:

alter database open resetlogs;


I highly doubt the above would work in your case, so below is the data loss scenario.

startup mount
alter database datafile 'V:\DB\CST001.DBF' offline drop;
alter database open;


Using OFFLINE DROP allows you to open the instance (as long as the affected datafile is not critical) without it trying to access that file. Since you can not restore and recover the datafile without backup, you can drop the tablespace (I am just guessing the name from the datafile):

drop tablespace cst including contents and datafiles;


Recreate the tablespace:

create tablespace cst datafile 'V:\DB\CST001.DBF' size ..;
alter tablespace cst add datafile 'V:\DB\CST002.DBF' ...;
...


Regrant any tablespace quotas if you had them before (check DBA_TS_QUOTAS before dropping the tablespace).

And import what you have in your Data Pump dumps.

The not so easy way would be trying to salvage data from the other datafiles in the tablespace before dropping it.