Sending data in processlist

[CD Ben]

I faced a strange problem in select query fro my application. While run a select query, see in peocesslist status 'sending data' and no result still 4/5 hours runnlig. Nothing will happend in my application.
my query like this:
select shutDownFlag, time_of_ping, observedstate, simulatedstate from test100 where time_of_ping = (select max(time_of_ping) from test100 where !(observedstate=4 and powercost=0))

Table have 20K rows (Size 4.5M)
MySQL Server version 5.5.22
MySQL Connecteor 5.1.11

Can anyone help me to solve the problem. I already spend 3 days on it.