you either have a bad control board or key panel but you can't get parts for this oven anymore,you'll have to replace the oven or you can try this because it sounds like the board is bad,either you or you get someone in to remove the board,you make a diagram of the wires,take out the board and send it out and have it rebuilt,it usually takes 10 to 14 days to get it back,then you reinstall the board and wire it.i've done this on older thermadore stoves,has the clock ever gone blank or anything crazy like that?if so then you know it's the board but i just googled it and these guys rebuild it for like 160 bucks and that ain't bad
https://www.appliancetimers.com/appliance?model=gbd307pdt7
call them,give them your model number or you'll have to replace the oven,this is why they told you to kill the power to it to see if the board would reset,you can try that again,leave the power off for like 5 to 15 minutes and try it but unfortunately they don't make your parts anymore.good luck