Zucchini often makes only male flowers at first, which then just drop off. The female ones are the ones with the tiny fruit already present behind the flower. If you don't have female flowers, you won't get any fruit. Make sure you have both.
Tomatoes often suffer blossom drop when under stress. Type blossom drop into the Search the Forum feature and find lots written here about it.
I'd say a little more, but Earth Hour is about to start! I have to go light some candles and turn the computer off!