How Do I Correct The Size Of An Oracle Datafile On A Database That Mounts But Doesn T Open

I’m getting the following error on starting Oracle database through the enterprise manager console :

ORA-01122 : database file 2 failed verification check

ORA-01110 : datafile 2 ‘D:\oracle\oradata\new1\undotbs01.dbf’

ORA-01200 : actual file size of 72733 is smaller than correct size of 72960 blocks.

The database mounts but is not able to open.  I don’t have any backup of the database nor the datafiles.

Please help.