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 The picture Above Is Showing You How To Take power way jumper on xperia x10,
If youve got fastboot and the fastboot drivers installed on your PC, connect your phone in fastboot mode, open command prompt, type "fastboot reboot" (without quotation marks), and it will turn on. You can do this after about 10-15 mins of it charging from the computer.

The next thing is to try is to plug your phone into the charger in case the battery is depleted. Some phones may even switch on this way but its unlikely so try long pressing the volume buttons in case a boot menu appears.

There are various things you can do to wake up your phone without using the power button. Try simply plugging it into a charger or getting someone to ring you.

The Xperia M  has a physical camera button, use this to launch the camera app then quit out.
For Rooted users, there is another way (The BEST way)... Change Power button to Volume button permanently! Open a root browser/ADB, then go system/usr/keylayout. There should be a file like mtk-kpd.kl (The file name varies from device to device, but it normally ends with -kpd.kl/-keypad.kl).

Change the power button to volume button there. For more information, search on XDA-Developers.
ADB doesnt work if phone is switched off (Doesnt matter if battery has charge), even if Debugging is enabled