Loss of power is one reason your AC may not turn on. Start by checking breakers in the main electrical panel to see if any breakers are tripping, if it is a breaker and it continues to trip, it could be an electrical issue or it could signify the compressor in the AC unit is failing. Fuses or breakers in the outdoor disconnect box can fail, causing loss of power to the AC. Break in control wiring from thermostat to AC could cause the AC unit not to turn on. Failed thermostat is another reason you may have no AC. If the system has tripped on high pressure due to clogged filters, or dirty coils it could need to be manually reset in the control panel at the AC unit itself.