1994 Other Toyota Models what would cause no compression in

Engine Cooling problem
1994 Other Toyota Models 4 cyl Two Wheel Drive Automatic

hi I have a toyota estima 1994 petrol engine, the car just died on us while driving home other night the temperature level never went above what it normally sits at had water in radiator,
next day could turn key and it would wind over but took ages to finally fire cough spluttered then went ok but after turning key of, had trouble starting it again, drove it up road to mechanics and it started for him first pop every time he replaced battery and then wouldnt start again.
he checked spark plugs etc then did a compression test and found I had none? Said that would mean the valves have gone, and had car overheated which it hadn't? As when car died opened radiator cap it wasn't even hot and water was level was full. So what would cause valves to go? Been told is going to cost heaps to fix now? Was fully serviced September last year also.