altering a db2 table with huge amount of data
Did you check the db2 diag log? what does it say?

The first thing comes into my mind it is that probably, the database does not rollback when the transaction log is full, but instead your updates never finishes.

Check the database logging parameter to see if there is enough log space to record the transaction.

BLK_LOG_DSK_FUL http://publib.boulder.ibm.com/infocenter/db2luw/v9r7/topic/com.ibm.db2.luw.admin.ha.doc/doc/c0006131.html