My Mackbook Pro 3.1 won't boot

Was using notebook last light and closed the lid when finished - sleep indicator was pulsing as usual.

Come to use it today and it wouldn't wake from sleep when opening the lid. No response from power button!

Removed power and battery, replaced, still no response from power button.

Checked for articles and tried the reset SMC fix - still no response.

Opened up the notebook - noticed the button cell not connected - replaced and re-assembled - still no power up!!

It's well used and well loved, but I don't think it'll be worth repairing - it had a new logic board 1 month after 3 year warranty period - that was a few years back

Any last suggestions as to bring it back to life? I'm confident in dismantling it to get to any potential problem/fix.

Thanks in advance!!