Have you tried to force shutdown by pushing the red/orange button at the back of your phone?
It is done by pushing the button for 5 seconds, after vibrating 3 times, your phone will completely shutdown as if you have removed your battery.
After that, you may proceed to enter fast boot mode and reflash your desired .ftf
Phone: Sony Xperia SP
Model #: C5303 (PM-0350-BV)
ROM: Paranoid Android v4.43 by neXus PRIME
Build Number: Build #8
Kernel: Williams Kernel v5.1
Samsung Galaxy Y S5360 (Handed over) > Sony Ericsson Xperia Mini ST15i (Sold) > Sony Xperia V LT25i (Swapped) > HTC One X Endeavor (Sold) > Sony Xperia SP C5303 (Recovered-Current) + Xperia M2 D2303 (Concurrent)