Recently, my MacBook stopped powering up. When pressing the power button, nothing at all happened. The usual green-yellow light on the power supply appeared normal, but no lights or sounds came from the MacBook itself. Only very rarely, after trying many times throughout the week, would it power on after pressing "power on". Once on, it never powered it off without me instructing it to shutdown.
I took the MacBook to a repair store. They said I should leave it for a few days. When I returned, it worked, albeit they lost a few of the screws, didn't completely bolt the parts firmly together, and got it really dirty. They refused to give me any details about what they did to fix it, but showed me that it works fine now. I don't know if they made significant changes, such as replaced internal parts, or just wiggled some wire and got it working again.
Are there any steps I can take to investigate the computer to identify how the repair store fixed?