Turn off your phone and let it charge for about 25 minutes, to make sure it has enough power to complete the following process.
After charging your phone, make sure it's turned off. On your computer make sure you have internet access, and go to Sony PC Companion, and plug in your Xperia L. Make sure it doesn't turn itself back on. Now in PC Companion go to Update Phone --> Fix Phone. PC Companion will promt you that you will lose all your data, Tick the box confirming you understand, and follow the instructions. PC Companion will download the appropriate firmware (which sometimes takes 15 minutes or more). After it downloads it, you will be prompted to unplug your Xperia L from the PC. Unplug it, then take out the battery, wait about 10-15 seconds, put the battery and cover back on, WAIT to make sure your Xperia L does not turn itself back on, and if doesn't press and HOLD the volume-down button, and without letting go of it plug in your Xperia L to the PC. DO NOT LET GO OF THE VOLUME-DOWN BUTTON UNTIL PC COMPANION PROMPTS YOU TO!
There is another method to factory reset if you don't want to use Sony PC Companion, but it won't work if you turned off your phone at any time after forgetting your pattern (or facelock, or code, etc). If you have not turned off your phone, then take somebody else's phone, call your Xperia L, and answer the phone when it rings. Once you answer it, swipe down the screen with your finger, and enter Settings --> Backup And Reset --> Factory Data Reset. That's it.
