iPhone 5 won't turn off

lcd on my iPhone 5 was cracked but touch screen was working fine. I was pre testing the new lcd replacement before complete installation. I turn off my phone, removed all three connectors very carefully and attached the lcd and digitizer cables, touch screen worked for few seconds opening up things even without touching. Touch screen stopped working then, I removed it and attached the old original one again and now it's also not working. I'm unable to turn it off by slider and I hold power and home button for 10 sec but nothing happened .

Today I ordered a lcd replacement from ifixit and need to know, how I would replace it without turning the iPhone off, since I tried everything to turn it off and why my old screen is also unresponsive while it was ok before.