Now what?

I have a 2/3 years old MacBook Pro, which is out of warranty. It was working fine. I got the “Software Update” notification and did it. After update, it immediately cannot boot correctly. I took it to Apple Store, and they said they could help since it’s software probably. Then, they couldn’t correct the issue and told me it’s the main logic board (or something like that), and it will cost $599.00. I believe it’s the MacOS and/or software, because it was working fine until the OS/software update. Has anyone experienced this issue and was able to resolve the issue? It would be a great help for me since I use it to look for a job.