run-away query

I am running a query in PGAdmin.
I changed the storage on a bytea field from External to Main and then ran
update dbmail_messageblks set messageblk=messageblk || '';
so that it would move all the rows to the new storage.
This is a very large table, so I actually expect it to take a long time.
The table has approximately 200,000 rows

select max(length(messageblk)),avg(length(messageblk)) from
dbmail_messageblks
MAX AVG
532259; 48115.630147120314

The query has been running for 6.5 hours now and in the status bar of
the query tool it says "Query is running" and the count in ms is still
running.

I had checked a number of times using select * from pg_stat_activity and
it was in there and running. The last time I checked the query was not
listed in pg_stat_activity. In fact There is no row for that window. I
have 3 windows open for that database, the one running the query, the
one I queried pg_stat_activity from and another one.
When I query pg_stat_activity it shows 2 rows for my ip address, the
pg_stat_activity and an IDLE row. When I close the other window it only
shows one row for my IP address.

I also checked in Linux PS - my IP address only shows up in ps aux for
the other windows, not for this 6.5 hour query.

Is there any way to check what is going on?

Thank you
Sim