I test my tablet SGP321 with many ROMs.. I end up going to boot loop... Crashes and stuff but I always recover from it.
The things I do before doing anything insane with my tablet
1. Take backup off apps/data
2. Always enable debugging and Unknown Sources
3. Keep the backup of the ROMs
4. Always have the xperia flashtool on my computer
Here's what you can do..
Press and hold the Power button and the Volume Up button for ten seconds until you see a white light flashing three times. While doing this somewhere around in 5 seconds you'll see white led flashing once, but keep on pressing those buttons till it flashes next three times.
After that your tablet will be turned Off.
You can put your device in flash mode and flash a new ROM from Xperia flash tool.
If you get the error "Unknown sources and USB debugging must me enable for flash mode, then your option can be to flash the ROM from fastboot.
Download a ROM (CyanogenMod or any with custom recovery).
Flash the boot.img via fastboot and reboot your device.
Once done you should see a led lit up while the device is booting (pink/magenta color in case of CyanogenMod).
Press the volume up/down button and you should be able to go to recovery mode.
From here you can recover your device easily
And before you do anything make sure you've charged your device well.