1. Install RSD Lite 4.9
2. Download and extract The right SBF restore file
3. Plug in your phone. Power it off and hold CAMERA + VOL DOWN
4. On the PC launch RSD Lite, Click on the [...] to the right of filename, browse for the .sbf file you have just extracted.
5. Click on Start
6. In normal cases the phone should boot up now. If you are SURE it doesn't, which is when it tells you to boot up the phone manually follow the next steps.
7. Unplug the phone and pull out the battery.
8. Slide out your keyboard and hold the X button, insert your battery and power it up while still holding the X button.
9. A screen with an exclamation mark pops up, press the @ button on the keyboard to open the recovery menu.
10. Use either the arrows on the keyboard or the VOL UP / VOL DOWN buttons to navigate up and down. Go down to wipe data/factory reset.
11. It will hang at some times but in the end it will say it's done. Hop over to reboot system now and reboot.
Congrats, if the problem was software-based, your phone will boot now!