Thermal Runaway, Instant heating up

[hun46y3]

Hey there!

After a nozzle change (nothing else changed on my printer) this started:
As soon as I turn the printer on, the hotend starts to heat up, and it just goes higher and higher until I turn the printer off.
The temperature reading on the display also increases, so it's not a thermistor issue.

Maybe a mosfet burend out, or something, and it just constantly supplies voltage to the heating cartridge?
Do you guys have any idea what to check, what to do?

Thanks in advance!