Contact Prusa Support via the Chat function and they will talk you through a safe procedure.  You will need your machine's serial number.

Cheerio,

Good evening, looking for a little guidance here. I'm not overly familiar with the prusa 3D printers. I had a lot of problems with my first MK3 and decided to give it another go. I'm having similar experiences with my second one unfortunately. 

I'm going to say 30 or 60 days ago. I noticed it was starting to throw errors with heating, the bed as well as struggling to maintain temperature. So I heating bed and the wires. I noticed a pretty bad kink in the ground wire connecting to the heating pad that must have happened during assembly or shipping as it was pretty old. So I replaced the ground wire and added a new connection to the bed. I came home today to see the bed thermal runaway alarm and the grounding wire melted in half. 

I'm assuming my next step should be to visually inspect the bottom of the heating bed, but thought I'd ask the community first