Hi there and welcome!
If you look at the back of your MES (
Masterbuilt Electric Smoker) and you are lucky... you will have a panel that is about halfway up the back either in the middle of the back or to the right hand side of the back.
Open that panel and remove any box you see under the panel.
There you will see the Safety Rollout Limit Switch. This thing is often a culprit causing what you are explaining. If there is a problem here you will likely notice the connector or the switch itself has burnt up, corroded, disconnected, etc.
Safety Rollout Limit Switch Replacement (if it's the issue)
If that is the case you can replace any corroded connectors with these (do this anyways):
If the safety rollout limit switch is burned up or damaged you can simply cut the connectors off the wire right where the connector and wire come together to leave as much wire as possible.
Wire nut the ends of those 2 wires together and wrap with electrical tape.
That will get you going but WARNING there is no extra over temp safety switch in the mix so always be able to monitor your smoker temp.
Get these EXACT replacement switches, do not get another one that looks the same simply click this link to buy the correct ones or you will likely get the wrong ones!!!
Once these come in undo the wire nut and use the Supco Hi Temp connectors to connect back to a new switch THEN install the switch into the MES.
If the safety limit switch isn't the issue then more trouble shooting needed like checking UNDER the MES in a panel to see if the lower controller board shows signs of issues. But one step at a time.
You will want to check all of this out before we start talking about replacing your controller as these are simpler and much cheaper fixes.
Let me know if this info helps :)