Hello I know this a couple years after this problem started Recently my Xperia Tablet Z did the same thing. Wasn't used for some time. I tried to power it up - nothing. Tried to charging it. Nothing. At some point red light came on and it didnt do anything doesnt matter if it was charging or not. I heard 3 x blinking red light means batter dead. After i tried every possible combination with buttons I gave up. Next step - taking it appart. Its pretty easy. The back cover comes of after u use small plastic pry tool...and go around the edge. It is glued but easy to peel off. I also used the heat gun (u can use blow dryer) to help a little (but i think that heat gun fixed the whole problem) While doing that heating up the back cover I have noticed that the red light is off. It seemed like the dead battery got little warm and started to work. I pressed the power bottom and now the red light was flashing 3 times (it never did before) So I stopped with taking the tablet apart and connected the charging cable. The red light came back on and my screen turned on showing that it is charging. Problem solved. I know its stupid but I think that battery got so low at some point of not using the tablet ( and the place was cold) that it needed little warming up. Hopefully that helps.
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