I had the same problem with my Xperia Z (yesterday!) - used phone until battery exhausted (I was using Google directions, fortunately I found where I needed to go, and part way home before the battery gave up). When I plugged in the charger, nothing. Left it on all night, nothing. When I pressed various buttons, I got the 3 red flashes, but essentially nothing. Plugged it into the PC, and ADB and it gave no devices connected. Got to reading these forums. Got the impression that Sony repair centres could fix the thing in minutes, so it couldn't be much of a repair. I had to replace the screen a while ago, so the back of mine is glued on with cheap glue I got from eBay. This glue is not a patch on the glue that Sony originally use, so I could get the back off quite easily. I disconnected the battery, not for long, probably only about 30 seconds max. Connected the battery, plugged the charger in and the red charging light came on. I left it a while until the white charging light came on, then started the phone with no problems at all. If it is something as simple as unplugging the battery and connecting it back up again, it does explain why it is such a quick Sony Centre repair!. A word of warning, if you've never had the back of your phone off, it will void any warrenty, it is a **bleep** of a job getting the back off, Sony glue is really strong, the glue will need replacing and it probably won't be waterproof afterwards.
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