My iPhone 4 will not turn on past the apple logo

Hello,

I replaced my iPhone 4 screen and digitizer as the previous one I had attached was cracked.

After putting all the screws back in there original places I fitted in the battery and screwed it down where it connects to the motherboard.

I placed the back cover on and screwed it in.

Over the next few hours the iPhone would randomly just shut off, and in order for me to restart it i had to take the back cover off, take out the battery and then put the battery back in again.

I am stuck as the exact same thing has happened again, but this time when i go to turn on the iPhone it will not load past the apple screen and after a few seconds it shuts off

I would really appreciate it if i could have some help on what to do to fix it.

Ben