。
The food has gone cold.
Literal
Cooking [subject-が] has-cooled-regrettably.
Combines 冷める (to cool down) with ~てしまう (regrettable completion) and ~ています (resulting state). The triple stack — 冷めてしまっています — conveys 'has unfortunately ended up cold and is still in that state.' The てしまう adds mild displeasure at the outcome.