Hi @pete27449,
Did you have both the main battery and the cmos battery disconnected at the same time?
If so, did you press and hold the laptop's Power button for a full 30 seconds before reconnecting the cmos battery and the main battery?
All the power has to be drained from the motherboard to reset the BIOS (in case it is corrupted preventing a normal start) and if you just disconnected the batteries it takes a fair while to do this. Pressing the power button forces the laptop to try and turn on (which obviously it can't), but in doing so it drains the power more quickly.
If still no good, then if the motherboard is an NM-B191 (printed on the motherboard itself), here are the [https://www.alisaler.com/lenovo-legion-y520-15ikbn-dy512-nm-b191-schematic/|schematics] that may help.