Laptop is dead after dissasemply

Hello,

Most probably I ruined my laptop but just want to get your ideas if any.

I dismantled my laptop because I was hearing a noise inside of it whenever I was moving it. I found out that the BIOS battery did fall of as it was being hold with a tape……….

When I added some tape I reassembled my laptop but does not work any more. I hit the Power on button but no lights, no fans. Its Dead. I am sure that I have put everything back in place. I have opened multiple laptops in the past. The only strange I noticed is that when I was disassembling it I noticed the back leds of the monitor to open instantly and then stop again…… without a proper reason. The power cable was unplugged but the battery was unfortunately on. Any tips/ideas to verify if the Motherboard is dead?