this just happened to me... white screen w/ logo, then blue screen then black and repeating... didn't realize how critical the order/placement of the screws around the efi shield is... here's what i did as a workaround... i reopened the phone, detached the home button cable to get to the efi shield, removed the shield's screws and the shield itself, reconnected the home button cable, put one of ifixit's thin plastic "guitar pick" tools over the three connectors so they would stay snug inside the phone (in other words, in place of the now-removed metal shield), then carefully snapped the phone shut and closed it up w/ the two bottom screws. and everything works!
re: 'blue screen' -- not a guaranteed workaround, but sometimes works to remove the EFI shield altogether if the order/size of the screws is forgotten/lost... Apple logo then blue screen
