A fused bulb is a bulb whose filament is broken. The filament is the main glowing part if it is broken the bulb will obviously not glow. Because the circuit will be broken.

A bulb works on the basis that when current is supplied to the filament it heat ups and this heating of the filament will make the filament glow like a glowing iron bar; that is for example when you heat a iron bar too much it turns into a red glowing iron bar.

So, the main working of the bulb is to supply the current to the filament it will get heat up and start glowing. But when the current increases too much, the temperature of the filament also increases highly. This increase in the temperature of the filament can break or melt the filament which results in breaking of the circuit. And this broken filament bulb is called fused bulb. And that is why fused bulb does not glow.