A few tips based on previous discussions:

• Ensure your car has fuel
  
• Check the voltage on your battery (some car stereos indicate or some fast phone chargers while they're connected to the charger socket)
• You can also tell if the battery is low if the car makes a sound as if it is about to start then fails (this can also be caused by other issues)
• Ensure your battery terminals are properly connected (not loose) and are not corroded
  
• If the voltage is low (11.7V and lower), try to jump-start the car (using another car or jump-starter kit)
• If the voltage is okay (11.8V and higher) but car won't start, ensure your key is being detected and is in good condition (no weak battery or faulty/missing transponder)
• If the key battery is okay and still not being detected then that could be loss of memory & the key needs reprogramming
  
• If you have an aftermarket security system it could have blown a fuse; get a mechanic to check it.
  
• If at this point, the car still won't start, you might have a faulty starter.
  

PS: Always start with the simplest solutions first.