During the replacement of the iPhone’s screen, the four screws holding the plate over the screen connectors are replaced. These screws are not all of equal length. If you fail to note which screw goes into which hole and try to insert a long screw in a hole meant for a shorter screw, it can damage the logic board of the device. This will cause an iPhone boot loop and a blue screen. Performing a DFU restore will not resolve this issue, it will result in an error. You can refer to this guide:
[http://www.hcqs.us/blog/fix-iphone-boot-loop-after-screen-replacement/|How to Fix iPhone 6 Boot Loop after Screen Replacement]