Most wall or central AC units are quite sturdy, and good for years or even decades of service. If it simply fails to turn on one day, the first thing you should check is the fuse or circuit breaker that supplies the unit. If the fuse is blown or the breaker is tripped, you have probably found the trouble. ACs can draw a lot of power, and if other appliances are running on the same circuit, the fuses or breakers are set to shut the circuit down before the wires can overheat and start a fire.
