The car has been overheating, there is an air pocket in the cooling system, and the car won't start.
If the engine has been overheating and got hot enough then you may have caused some serious engine damage causing the engine to no longer start. If it did not get too hot then these may be separate issues. When the coolant level is low from a leak or from a coolant change it is common for the engine to trap air in it. The best way to get the air out of this engine is to jack up the front end of the car about five inches while it is running to help release the air. As for the starting issue, depending on if the engine is cranking or not, you may have a failing battery or a crank sensor problem. You really should get the help of a certified mechanic, like one from YourMechanic, to diagnose both of these problems so they can get you back on the road.
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