Is it possible the phone was low on battery power when you did the upgrade? Try plugging it in to your normal charging outlet or to your PC and see if you get a response.
When I did the upgrade today, my phone failed to turn on at first. But instead of a red light, I had a green light. It took a few minutes of pressing and holding the button to eventually get it to respond. I get the feeling it just took awhile for the phone to get itself together after the upgrade. In the end, after holding the button down a few times, it did eventually turn on.
After I got it to vibrate and turn on though, I ended up with a strange camera icon on the screen after I did the upgrade and that stayed on my screen for several minutes. I waited and waited, but it wasn't really doing anything.. so I plugged it into my PC and the phone suddenly came to life and I got the Sony Ericsson splash screen. I'm not sure if plugging it in was the solution, or if it was just a matter of time. Either way, it panned out int he end.
If it's been awhile, and if plugging it in doesn't help, you may want to try to use SEUS to install the update again or repair the phone.
