I recently tried to do a screen repair on an iPhone 4.
It won't boot fully, so I may have managed to break something else. Basically it wont turn on, and I only get a flashing Apple logo when plugged into a computer. I believe it could be the motherboard, although I have visually inspected it extensively to determine if i could have physically broken anything. I could not detect any fault. When I use the suspect motherboard on another iPhone frame I get the same problem.
Get into DFU mode and do a restore in iTunes.
For DFU mode:
Nothing will show up on the screen, but if you connect it to iTunes it should recognise it. Then do a full restore.
If that doesn't work, you screwed something up inside, you might want to open it up again and re-check all connections.
I had the same issue when a car ran over my partner's 3GS: I changed the broken screen and front glass and all it did was flash on and off with the Apple logo and get hot.
My solution was to re-check all connections, start iTunes on my laptop, plug in the phone, hold the power and home button for 15 seconds, and (eventually) it'll go into recovery mode (this took me about 6 tries).
try to change the battery. it worked for me
More than likely you ripped the battery terminals off the motherboard. When connected to the computer it tries to boot but fails to detect a battery. Look very carefully at where the plug attaches to the board. You'll see one or several little brown patches where terminals for the plug to plug into were once attached. Look at the battery plug.....that's where they now are.
I attempted a battery replacement on my daughter's iPhone 4. After a bettery remove and replace (not the R&R we all hope for!) her phone did similar things as yours.
Check the connector on the main board your battery plugs into. Be really careful when you do this and don't remove the battery again if you don't need to. During the R&R, I managed to break the surface-mount solder joints and remove the main board side of the connector with the battery lead. I don't know if the connection was a pre-existing condition, or if my removal technique was sloppy or overly forceful.
I rectified the condition by upgrading my 4s to a 5 and giving her my "old" phone. If you're eligable, now might be a good time to consider a carrier-subsidized upgrade. If you're not, look for a friend with a 4s who is eligible for an upgrade but hasn't exercised it yet. See if you can pay for their upgrade fee, getting them a new phone and you a $200 replacement.
Good luck!
I had the same problem when I did a screen repair if you replace the screw in the EXACT place you removed them from the phone will no work properly ie no wifi, not charging, no call ect I hope this helps you with your problem :)
Have you tried DFU mode and restore via iTunes? DFU mode is turn off > home + sleep 10 seconds > release the sleep button, until a "connect to iTunes" appears.
My iphone 4 is doing the same thing. My diagnosis came down to this... just like my iPhone 4, your iPhone is broken and need a hardware repair.
I tried this DFU crap 4 times and got unknown error occurred (21) each time.
From my experience, I'm pretty sure unknown error (21) is apple code for completely "effed".
