This is the most common reason. If your Lava Iris 702 refuses to turn on (shows a black screen) or is stuck in a "Boot Loop" (constantly restarting at the logo screen), the system partition is likely damaged. The flash file can rewrite the partition and "unbrick" the phone.
Sometimes, a bad software update can corrupt the modem files, leading to "No Service" or "Unknown Baseband" errors. Reverting to or reinstalling the correct firmware version can restore these files, allowing you to make calls and use mobile data again. lava iris 702 flash file