Hi there and welcome!
I'm positive we can figure this out and confident it's a simple fix.
The likely culprit is your roll out safety limit switch. It's wired in so that if your smoker ever just kept heating and got to hot it would cut the power off to the heating element.
This area is one of the major failure points on the MES because
Masterbuilt uses the crappiest connectors known to man hahaha. So the connectors have gone bad and/or the switch itself has melted down.
This is the switch:
Inside, looks like a circle around this area of the inside back wall.
View attachment 516724
If you are luck the back of your smoker has a panel and you can get to it. If not you will have to pull the back off or cut a panel back there without cutting too deep into the metal to cut insulation and wires if they are around.
View attachment 516725
Once you get to the switch from the back of the smoker it will look something like this. It can also be horizontal instead of vertical.
View attachment 516726
So the connectors have corroded and fell apart. And/or the plastic switch has melted.
If the switch is failing you can cut the 2 ends, wire nut together, and wrap in electric tape so if it comes loose you don't hit metal. Simple.
Here's high temp connectors that are much better than the trash MB uses.
Here is a replacement switch... GET THIS EXACT ONE! There are two different styles that look the same but are not the same so get this exact one if you want to replace a melted down one. Also these things are super delicate. If you wiggle the tabs at all from the back of the switch it will fail on you within months and you will be back at replacing it again.
For now to get you up and running you can wire nut those two ends together and bypass the switch but understand you safety switch is gone. Chances are you never will need it but it's there for a reason so it's up to your discretion.
Now if you want to turn this MES into a beast you can do a simple rewire (cut 4 wire ends, splice to make 2 wires) and use what is called a PID Controller ($150) and it will be like turning a golf cart into a Ferrari!!!!
Best electric smoker out there is an MES you get for cheap or free, rewire, and throw a PID onto. It's 50X better than anything
Masterbuilt makes brand new and you will keep yours running forever once you change those connectors to hi temp ones and with the fact that you have a backup heating element should your new one go out :)
I hope this info helps :)