My DEF sensor was melted and I can't get over 50, in my ford escape, what do you think is the problem?

My DEF sensor was melted and I can't get over 50, in my ford escape, what do you think is the problem?

Added (1). I replace corails and plugs but, my DEF sensor was melted and no my ford escape can't get over 50 mph

What's a DEF sensor?