Suppose the price elasticity of demand for oranges is 0.8. if a fall frost destroys one-third of the nation's orange crop, how will that affect total revenue from oranges, all other things unchanged?

Total revenue from oranges will fall. Notice that the question assumes everything else unchanged. This means that even though the quantity has been reduced by the frost, the price is unchanged. Thus all producers are selling fewer oranges at the same price. It logically follows that total revenue will fall.