Lack of power, a bad power cord, failed drive motor or broken wires in the vacuum hose can prevent the vacuum cleaner from starting.
Make sure the electrical outlet that you're using for the vacuum cleaner works. Try using a different outlet if you suspect that the outlet you're using is dead. If the vacuum cleaner still won't start, make sure the outlet has power by plugging a lamp or small appliance into the outlet. If the outlet is dead, reset the house circuit breaker for the outlet. If the outlet still won't work, have an electrician repair the outlet.
If the vacuum cleaner won't turn on and you know the electrical outlet is okay, check the power cord and replace it if damaged.
If the suction motor won't run when activated, replace the suction motor.
In a canister vacuum cleaner, check the vacuum hose for damage because the hose has integrated wires that carry current from the power switch to the suction motor relay. A torn hose can cause a break in the wires-interrupting current traveling through the hose to the suction motor relay and preventing the suction motor from starting. Replace the vacuum hose if it's torn or damaged because wires inside the hose are could be broken.