Howdy,
Technical Specialist Ekield here!
Sounds like you were having some trouble with Overwatch and your PC rebooting. I definitely know that it is never fun when you run into issues like this, and I want to let you know that we will do everything in our power to help you get it resolved.
When your PC reboots it can be a sign of a couple different things: overheating, driver/software conflicts, Windows corruption, or failing components (power supply, CPU, GPU, etc)
Your temperatures sound good (although 80C on the GPU is about as high as you'd want it to go), so I don't believe the issue is overheating.
Try running the ScanDisk and Windows File Checker to scan and repair the core Windows files:
https://us.battle.net/support/en/article/629
I would also suggest trying Selective Startup so you can shut down background processes and services. This will help determine if it's an issue with a driver or software conflict.
https://us.battle.net/support/en/article/200483
Finally, depending on how old the power supply is you may find that the power supply is starting to fail. This is a common cause of PC rebooting when the system is under stress (such as playing an intensive game like Overwatch). You may need to take your PC to a local repair shop or somewhere that can test your power supply and other hardware.
As for the Hearthstone code issue if you can provide the code we can take a look to see if there are any issues with it.
Hopefully this will help out. Have a good one!