Get the Xperia flasher from here first.
Get one of the the stock ROM ftf file from xda-developers .
Steps
1. Turn Off the phone.
2. Open the Xperia flash tool.
3. Click the lightning icon and Choose the Flash Mode.
4. Choose the folder containing the ftf File.
5. Now you will be prompted to Connect your phone.
6. While holding the vol- key connect the data cable.
7. Now the LED will glow green and your phone is connected in the Flash Mode.
8. Wait untill the flashing operation is complete .
9. Disconnect the device and turn On to boot back to the stock Rom .
