Why does it do this and what can

So i have an iphone 4 i changed the battery and switched phone back on, no apple logo came up just dim blue screen can hear the phone noises, no picture, can't get on the phone at all tried rebooting and resetting phone bu no joy but pics it up on computer, restarts on that but nothing comes up on phone still. It syncs phone on itunes so the phone must be working just a dim blue screen on the phone… Please help?

Sounds like you forgot a step when putting your iPhone back together. There might be a ribbon cable still unplugged or you might of damaged something… Hopefully not though.

Here is a link:

Presumably you must have damaged the screen or connector when you changed the battery
HTH
David