I just figured it out...FINALLY. (you must know that i repair mobile devices all day long. I have worked on over 500 iphones, 200 iPads and 400 iPods..So i know what i am doing). This repair took me 6 hours...i had the screen off within 5 min. I found that if i hold my thumb over the solder point of the battery cable/ Logic board with pressure..the iPod turned on. When i took my thumb off...it shut off. I inspected the solder and the cable and everything looks good. I put some electrical tape over it and layered it in 3 layers..then put the Metal plate/sheild back on and im good to go know.. Dear iFixit: Change your instructions and have people De-Solder the Battery...the cable and solder points and or pads are very fragile...I almost lost a customer because of this.

The same thing just happened to me...I am working on on a touch 5th. I just replaced the front screen and now the thing doesnt turn on. All Cables and solder points are intact. Not sure what to do at this point..