NullPointerException in prepareStatement

[mangesh lele]

could someone please tell me what are the reasons why a null pointer exception is thrown in a Connection prepareStatement(String) call?
I am generating the required insert statement given below and passing it to prepareStatement :
Insert into o1pay.product_pay ( AMT_LMT_DEF_CCY,.........40 odd fields ) values ( ?,........same number of question marks )
the logs are as follows:
java.sql.SQLException: java.lang.NullPointerException
at weblogic.jdbc.rmi.SerialConnection.prepareStatement(SerialConnection.java:86)
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furthermore, this error started happening when i moved my weblogic application setup from one database to another database, apparently identical to the first. the only thing i changed was the database url in connection pool tag of config.xml (weblogic 6.1)
thanks in advance