A black hole can be considered a star that has

collapsed in on itself

begun to form new stars

changed into a brighter star

disintegrated into stellar dust

The first option, collapsed in on itself. The star's core mass becomes so dense that the resulting gravity implodes the star. Interesting enough, the third option is kindof true too ... some large and tenacious black holes that absorb other stars will form incredibly bright accretion disks around their perimeter before filling absorbing the star.