B767-300 First Officer reported a loss of thrust on the right engine after reaching cruise altitude. Flight crew was unable to restart the engine which had an MEL for the right thrust reverser being inoperative and landed uneventfully at departure airport.

Narrative

This was a standard Company cargo flight from ZZZ to ZZZ1. We operated this aircraft from ZZZ2 to ZZZ and were continuing on to ZZZ1. The right thrust reverser was deferred and locked out. We had the associated ICAS indications. We departed just after sunset; it was warm in ZZZ 38 Celsius on the ground. There was no significant weather in the area; it was a generally clear VFR evening. The Captain was the pilot flying and I was monitoring from the right seat. We departed and climbout was normal. The departure was busy with several ATC assigned step climbs and vectors. We were given a fix direct ZZZ3 and eventually given a climb to our cruise altitude of FL350. We leveled off at FL350 and I went heads down to calculate the en route fix times. The Captain alerted me that the right engine had rolled back and was not producing thrust. He then directed me to request priority handling so we could start a drift down and turn toward an airfield. We discussed briefly where to go and considering our altitude and time to descend we turned toward ZZZ and ZZZ4. The Captain then called for the QRH and I accomplished procedure as we did not suspect severe damage. At this point we determined a restart was warranted and accomplished QRH procedure. The restart was unsuccessful and we elected to setup for a single engine approach and landing. I setup the aircraft for the approach. At this point we were over ZZZ and we overflew the field and entered a downwind to finish completing the briefing and checklist. We then flew the approach and landed without further incident.Root cause is unknown to me; I suspect it had to do with the right reverser lockout. There was no visible physical damage.