if you already change the charge flex check very carefully the conector in the mainboard looking for any damage and try whit another flex.

You can solved the issue in two ways 1 making a hard reset (pwr + vol up) and make all the wipes ( data, cache and dalvik cache if you got acces) 2 try to reinstall de alarm apk corresponding to your android OS version If don´t want to download all your apps again here is this https://play.google.com/store/apps/detai... its work perfect to me .

Only by restoring you can have your phone back. If you have an itunes or icloud backup you can retrieve the info you saved, but all the info you haven't backup will be lost. To recover you phone turn it off and plug into a pc or mac while holding the home button and follow Itunes indications

Check the batteries voltaje (both de new and the old) to be sure that they are at least charged. If the batteries have at least 3.5v leave the phone charging by 30 minutes, and check again, if you got the same lecture or less the issues is in the charger port. Also if you have an external power supply charge you battery until they are in 3.9v o above and try again to turn your phone on.

Check the back panel of the screen, sometimes just reheat that part solve the problem. If this don´t work check for any damage in the components or broken solder.