I can't switch my sony ericsson K810i on?

I've got a Sony Ericsson K810i and I was using the sim card out of my phone this to put in my last phone to delete all the info as I was about to sell it. This morning when I put the sim card back and then tried to switch it on there was nothing at all, not even the tiny little buzz when you first switch on. I left it on charge all day even though I knew it wasn't the battery but still nothing. I went to a phone shop where they very kindly tried one of their batteries but still the phone just won't switch on. How could I have broken the phone just from removing the sim card and battery and then putting them back? Has anyone come across this with a sony ericsson before? Please help????