How to recover my database in DBF?

hi everybody,

i’m getting the following error on starting oracle database throught 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 databse nor the datafiles)

plz help

thanks…