Broken power cable after trying to flash new firmware

Hi 3dprintboard-community,

Two years ago I changed the motherboard of my 3D Printer ANET E10 to a new version (V 1.7). The printer ran quite fine.
The power supply dlivers with 12V, since I live in Germany I set the power supply to 220V.

Since I wanted to include the Mesh Bed Leveling function, three days ago I burned a bootloader on my board via USBasp and afterwards updated to the new Marlin Bug fix 2.x via USB.
The flashing went well, but after having installed the new firmware, while heating up the hit end, the error "Thermal runaway, E1 heated..." popped up.

Since I couldn't resolve this issue I first wanted to re-do buring the bootloader via USBasp, which seemed like it worked.
But suddenly I couldn't flash the firmware on it, since the printed did not respond.
After a few tries, I reconnected the power cable to the printer. It turned on and suddenly off again.
I checked the fuses of the motherboard, but it seemed unharmed. Only on a corner of the cardridge part of the motherboard it seems a bit "brown".

Then I realized, my power cable is not funtional anymore, which is the reason for turning off.

In the motherboard Now my questions:
- What did I do wrong?
- Can I just plug everything back in with a new power cable?
- What do I need to do to prevent it to happen again?
- How are my chances that my 12V power supply is now damaged?

Thankful for every support here!