Imac G3 400 Dvd Rom. With Macosx Tiger Wont Boot!??


i would like to know? why macosx tiger starts but wont boot.. its IMAC G3 400 memory ram 320 MB.. with dvd ROM.. i have installed MACosX tiger, and it used to boot but recently its blocked it makes the startup sound but nothings happening!!? what to do??? thanks