To force rebooting the phone:

• unplug the phone (= this is important because on Xepria, the button presses have completely different meaning when the USB cable is plugged in. Remember back when you flashed it ?)
• Hold the power button and the volume up button
• Wait until the phone vibrates (a few seconds, on my X it's about 2-3 sec, on some other Xperia models it might be longer like 8-10 sec)
• Now you can release the buttons
• The boot loader should display the usual "Warning: the phone has been unlocked", and the kernel should display the usual "Sony" logo while booting.
• Now you can plug the phone back in

If that doesn't work, you should try booting into recovery mode:

• If you haven't flashed a sailfish recovery kernel, you need to reboot the phone into fastboot mode and load the recovery kernel from a computer
• IF you have flashed the recovery image you can boot it with the volume down key

Then from the recovery mode, you can telnet into the smartphone, try to copy any precious data onto the sd-card, and attempt to repair any corrupted/damaged filesystem.
Or straight recover/reflash the phone (hint: you have clean new partition stored in /fimage that you can use to rewrite the corrupted one).

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And here's an extra link to yet another TJC answer on the subject of recovering phones.