Query is stuck

I have a query which is not giving me the result even after 30 minutes.
I want to know how to detect what is going and what's wrong ?

EXPLAIN query - gives me the following:

controlsmartdb=# explain select report_id, dm_ip, dm_mac, dm_user,
dm_os, report_time, sys_name, sys_user, sys_user_domain, ss_key,
login_time, role_id, new_vlan_id from repcopy as a where report_time =
(select max(report_time) from repcopy as b where a.dm_ip = b.dm_ip and
a.dm_user=b.dm_user and b.ss_key != '') and report_status = 0 and
dm_user = 'u1';

QUERY PLAN

------------------------------------------------------------------------
----------------------------------------------------

Seq Scan on repcopy a (cost=0.00..1630178118.35 rows=35 width=133)

Filter: ((report_status = 0) AND ((dm_user)::text = 'u1'::text) AND
(report_time = (subplan)))

SubPlan

-> Aggregate (cost=8151.65..8151.66 rows=1 width=8)

-> Seq Scan on repcopy b (cost=0.00..8151.65 rows=1
width=8)

Filter: ((($0)::text = (dm_ip)::text) AND (($1)::text =
(dm_user)::text) AND ((ss_key)::text <> ''::text))

(6 rows)

But EXPLAIN ANALYSE query hangs (is not giving me any output even after
30 minutes).

Pg_stat_activity shows this - SELECT procpid, usename, current_query,
query_start from pg_stat_activity:

2942 | postgres | select report_id, dm_ip, dm_mac, dm_user, dm_os,
report_time, sys_name, sys_user, sys_user_domain, ss_key, login_time,
role_id, new_vlan_id from repcopy as a where report_time = (select
max(report_time) from repcopy as b where a.dm_ip = b.dm_ip and
a.dm_user=b.dm_user and b.ss_key != '') and report_status = 0 and
dm_user = 'u1'; | 2010-04-13 18:20:02.828623+05:30

In such a case what can I do ?