What happens after the first stage separates?

what does the first stage of a multistage rocket do?

what happens when the first stage runs out of fuel?

what happens to the second and third stages when they run out of fuel?

When the first stage separate, then it will not descend further and it will fall away. It would not be able to launch into air.The first stage actually help to release it from shell, the first stage of multistage rocket help it to sprout.If the first stage runs out of the fuel, then there will be no combustion in rocket and it will not ascend further into next stages.If the second and third stage run out of the fuel then the next 4th and 5th stages will happen to a cessation and there will be a likelihood for it to fall down to earth.