I have had the same problem, and it turned out that my battery cable was not connected.
I lost my battery cable-metal cover and i dropped my iphone. I think that what caused it to disconnect.
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So if you had your phone opened previously or replaced anything like the screen or the battery or anything by yourself or by someone else, you may want to check on your cables again.
I have had the same problem, and it turned out that my battery cable was not connected.
I lost my battery cable-metal cover andi dropped my iphone. I think that what causes it to disconnect.