Problem with developer 6.0

[Sllewnerak]

I am having a problem with a form I have developed. I am just learning forms
so I don't know what to do about it. I have one form with several blocks in
it. When i first run the form I can go on any of the blocks and go through
the records one at a time. Then all of the sudden it won't fetch any more
records and I can't get anything to work untill I close it and recompile it.
What makes no sense to me is that it can happen at any point in any of the
blocks and then the next time the block that didn't work last time does and a
different one won't. These are the error messages I am getting.

ORA-01041: internal error. hostdef extension doesn't exist

FRM-40507: ORACLE error: unable to fetch next query record

ORA-03113: end-of-file on communication channel

If anyone can help me with this I would greatly appreciate it. I am trying to
learn Oracle forms from a book and it is proving to be quite difficult.
Thanks, Karen