Answer
The 25W light bulb burned out.
Work Step by Step
The 25-W bulb has a resistance that is 8 times greater than the other.
The resistance can be written in terms of power and voltage.
$$R=\frac{V^2}{P}$$
Both bulbs are typically operated at the same 120 volts, so the lower power bulb filament has the higher resistance.
When the bulbs are in series, the current going through them is the same. The bulb with the 8 times higher resistance, the 25-W bulb, will have a voltage drop that is 8 times greater. $V=IR$.
It is subjected to far more than half of the 240 volts that is applied, so it burns out first.
(A deeper calculation shows that the voltage drop across the 25-W bulb is 213.3V, while it is 26.7 V across the other.)
