MacBook won't turn on, battery dead?

I found our old Macbook Pro in the cabinet, and tried turning it back on to see if it could still be salvaged. Upon pressing the power button, I get no sounds from the fans, so I presumed the battery was dead and needed charging (considering it hasn’t been used for almost 5 years). I plug it to its charger (and that to the wall outlet), and get a steady green on the MagSafe connector, and I also noticed the battery indicator on the side of the device blinks five times. I’m not entirely sure what this means, but I’m assuming the battery is dead.

Does anyone have any ideas on what I can do to check what’s broken before I go on and buy parts to replace for this? Thanks in advance.