I’m making a cheddar. All was going great until today, checked the cheese and both the top and bottom are bright illuminous yellow. Is my cheese ruined?

This sounds like a bacteria called Pseudomonas Fluorescens, which is a water contaminant, and produces a fluorescent yellow colour action when it grows. It is a spoilage organism and will rot the cheese. If on the ends, then you can trim that off and retreat to get a good rind. The growth of P. Fluorescens indicates the humidity of the ends of the cheese has been too high, storage on a rack, rather than a solid shelf will help, as will turning more frequently.