unhandled user-defined exception

[Venugopal, R (GEP, Contractor)]

Title: unhandled user-defined exception

Hi Gurus 
 
   I am getting the error attached below when trigger fires. what could be the reasons
for this error.
 
DB error: ORA-06510: PL/SQL: unhandled user-defined exception 
 
 
Thanks in Advance
Venu