I had this problem right away with my EX500-40" and it drove me crazy for a week until I found this solution.
1. Turn TV off on the unit (use the power switch on your Bravia set, not the remote). (and see below, I'd suggest before turning it off, set the input to the HDMI input where you have the cable box plugged in. NOTE: on my Bravia, I had the cable box hooked to HDMI 1, pressed Input to select HDMI 1, and it did not say "no signal" but rather just showed a black, blank and soundless screen). 2. Unplug Bravia set. 3. Wait 30 sec. 4. Plug Bravia set back in. 5. Push power button on the Bravia set, and keep your finger on it pressed in - hold for 10 seconds.
I had my Bravia "input" set to HDMI, where my cable box was hooked up, and presto, about 8 seconds into holding the switch the HDMI input was recognized and the cable was on. Note also, my cable box was powered on the entire time as well.
Now we'll see if this "sticks."
Found this solution buried elsewhere but alot of people had a "Eureka" moment so I decided to try it. I messed it up a couple of times due to vague instructions there, but the above is exactly the steps to follow.