The flowers and buds turn brown, wither and die.
Late frosts can seriously damage flowers especially if it has been preceded by warm weather so that there is new, tender growth which is easily damaged. Rapid thawing as experienced by plants that catch the early morning sun after a frosty night exentuates the damage. The severity of the damage depends on how fierce the frost and how tender the plant is. A young tender plant can be killed by frosts, or only a few flowers will be discoloured. If the flowers of a fruiting plant (such as apples and pears) are very badly damaged they will die before being pollinated and no fruit will be produced that year.
Once a plant suffers frost damage, all you can do is remove the frosted parts. In the future, avoid planting susceptible plants such as early flowering camellias, magnolias and rhododendrons where they might catch the morning sun in spring. Tender plants can be protected from exceptionally late frosts by covering with a double layer of garden fleece. If a late frost catches you out, you can try to minimise the damaged caused by covering affected tender plants with sheets of newspaper to slow down the thaw the following morning.