os crash and not able to install new os

I had charged my battery to full. next morning when i woke up the phone was switched off , i tried to restart it but it stops on the logo and then starts again .same thing happens continuously. it keeps on reaching till the logo point and then restarts. can you please recommend a software which can help me to recover my data from the phone for free?and also please write about installing new os in my phone from my computer.

The computer is not able to detect my phone.