Ashes cannot be burnt because they are the solid remains of something which was burnt.

Burning something means breaking chemical bonds in a material and forming new bonds with an oxydizer, which most often is oxygen. Some materials burn entirely to gasses and vapors, leaving no ashes behind. For example, hydrogen gas combines with oxygen to form water vapor. Methane and other hydrocarbons, which are composed only of hydrogen and carbon, combine with oxygen to form water vapor and carbon dioxide. Wood, paper, and many other materials contain elements which form solid residues when burned. These elements include silicon, calcium, and iron. Ashes are the portion of a burned material which does not turn into gas or vapor. Ashes typically include oxides of silicon, calcium, and iron.

Most ashes can easily be turned into something else by being eaten. After a forest fire, microbes consume the ashes and turn them into dirt, in which plants grow. Plants absorb the waste products of the microbes in the dirt through their roots, and turn it into new wood, or leaves, or fruit.

-- Jeff, in Minneapolis