My cucumber flowered and you could see tiny cucumber all over now they have gone yellow and almost disappeared. Lots of sunlight, watered and something for them to grow up.

Poor pollination, are there plenty of bees around to pollinate the flowers. The plant drying out will cause fruit to abort. A sudden change in temperature, too wet during pollination. Give your cucumber a helping hand by cross pollinating the flowers. Fruit dropping off is a sign of the plant being in some sort of stress.