Lumia update unbrick
1) Download NSU 4.3 for Retail free from here (about 104MB) and install in your Windows PC (Win7, 8 or higher) - it will take some time as many components will be installed
2. After installation, launch NSU - you must be online to use NSU (a fast internet connection is recommended) - you should see following launch screen
Nokia Software Updater 4.1 Retail
3. Now there are two scenarios
A. If you have a normally working phone - just connect it with a supported USB data cable and wait for a while (do not click "My device does not boot up") - NSU will detect the device and check the latest available software for it online - after some time NSU will show you the latest phone software details as shown below
Now click "Update" button - wait as NSU will start downloading firmware files (download time depends upon phone model and your internet connection speed) - after download finishes NSU will start flashing the phone and will notify you when successfully done!
B. Dead/bricked phone flashing - if your phone is having problems booting up (completely dead, restart loop etc) due to whatever reason (except hardware issues), then at "Connect your device" screen do not connect your phone - click the "My device does not boot up" button - NSU will forward to another window as shown below -
Nokia Software Updater
Now connect your phone (preferably in switch off condition) with computer using USB cable - if phone is a Nokia Lumia device then press and hold Volume Down and Power button simultaneously for a few seconds - phone will vibrate and NSU will detect it and after a while will start downloading firmware files
now all you have to do is wait, wait and wait as downloading over 1GB data (varies depending upon phone model) will take time and then flashing will also take some time. NSU will notify you once the process finishes, done! Back to life your Nokia phone.
Note - Although specifically designed to solve bricked Lumia phone issues, this version of NSU works perfectly well for other Nokia phones too (Asha, S40 and Symbian).
Supported Nokia Lumia phones (WP 8/7.5/7) - Lumia 800, 900, 510, 610, 710, 520, 620, 720, 820, 525, 625, 920, 925, 928, 929, 1020, 1320, 1520 etc)