If you are new to the XPERIA SP, you may find the below issue. Since I resolved the issue using assistance from the information highway, I thought I should write a small guide about the same. The XPERIA SP is a brilliant device. It is fast, trendy, and ready for future technologies.
Below is a snapshot of XPERIA SP -

Below is the description of the symptoms for the problem, and the solutions for the same -
PROBLEM - Phone turns off, and refuses to start.
1. Power button does not wake up the device.
2. Power cable plugged in does not wake up the device.
3. Cable connected to computer/ laptop does not power up the device.
NO WORRIES! - Your device is hibernating like a bear, and needs a NUDGE!
SOLUTION -
Open the back cover of the device.
Locate the yellow button 'OFF'
Use a ball pen nib or blunt pencil nib, and gently press the same and hold for 10 seconds.
After 30 seconds, power on the device, and it should be working just fine.
Below is a snapshot of XPERIA SP -

Below is the description of the symptoms for the problem, and the solutions for the same -
PROBLEM - Phone turns off, and refuses to start.
1. Power button does not wake up the device.
2. Power cable plugged in does not wake up the device.
3. Cable connected to computer/ laptop does not power up the device.
NO WORRIES! - Your device is hibernating like a bear, and needs a NUDGE!
SOLUTION -
Open the back cover of the device.
Locate the yellow button 'OFF'
Use a ball pen nib or blunt pencil nib, and gently press the same and hold for 10 seconds.
After 30 seconds, power on the device, and it should be working just fine.