Chosen Solution
I replaced the screen on my iphone 7, later realising that I did not disconnect the battery. When i tried to power it on nothing showed, but when I reconnected the old screen it came on but stayed stuck on the apple logo, what could the problem be?
It’s very important to always disconnect the battery before disconnecting/reconnecting the screen assembly. The issue is that the LCD connector has pins that have high voltages (i.e. the backlight is ~20V) and ground in very close proximity. When you fumble around trying to line up the connector plug to the receptacle, you can short out the voltage rail and cause damage to the components in the backlight circuit. If you’re lucky, then only a backlight filter is damaged but other times, the entire circuit could be affected. This can also cause boot issues depending on the severity. I would start by testing a bare-bones setup. Start by disconnecting the battery first (always!). Then disconnect the digitizer, LCD, front camera, rear camera and home button extension flex. Now re-connect the battery; you should only have the battery and Lightning Dock flex connected. Connect a known-good, preferably Apple-original Lightning cable to an iTunes enabled computer. If the bare-bones setup does not get recognized by iTunes, then you most likely have a logic board issue. You could send it (or find a local) to a repair shop that specializes in micro-soldering repair.
