The most possible cause is the main relay(located behind the dashboard near the driver side left kick panel) , or the distributor assembly and/or pickup coil. To diagnose the main relay simply put the key in the ignition, turn to position 2 (On but not cranking), and wee that all the dash lights are on. Locate the check engine light. Make sure it turns off after approximately 2-3 seconds following a audible "click" of the relay after you have turned the car on. If the relay is not functioning it will not send a signal to your ECM therefore cutting off spark and/or fuel. Secondly in order to diagnose the distributor assembly. Simply put on a pair of gloves(to avoid grounding yourself and getting shocked!) carefully remove one spark plug wire from the spark plug side. Then put it near any metal part of the car and have your buddy crank the car for you, watch for spark coming from the tip. If you cannot get a spark since your metal is too far into the wire boot. stick a screwdriver in and position the screwdriver rod close but not touching to something metal. If still no spark then you need a new distributor. I hope this helps!

Do you mean that the engine starter does not function? The term "not start" has two meanings and you have not supplied enough other details to narrow down which one you mean.

Do you mean that the engine stumbles like it wants to stall? If so,get a code reading for free at AUTO zone or other and come back.

thermostat is broken. because the water does not flow properly and the temp difference occur. and take the car to an electrician in order to check the relay of the fan or the motor of the fan. Usually is the motor of the fana and die suddenly

It is the mechanic, probably did not set the timing correctly or damaged the crankshaft sensors. But are you sure both sensors are damagedb because If both were dead the car would not be able to start,

more info? difficult to start or does not work properly?

check cooling fan operation, or you might have a restricted radiator.

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other stuff can make it overheat too like maybe the fan not kicking on or the cooling system needs to be flushed, especially the radiator.

check the radiator hoses to see if they feel spongy...you should not be able to squeeze the hose fully at this time. If you can scrunch the hoses to where both sides touch... http://www.spobd2.com/ you have a pressure problem - which could be a bad radiator cap.

Get a scan done for free AUTO. Zone etc and see what code it has .Then it is easier to find the problem.

Here are a list of conditions that can cause an overheating condition;
A low coolant level, a coolant leak (through internal or external leaks), poor heat conductivity inside the engine because of accumulated deposits in the water jackets, a defective thermostat that doesn't open, poor airflow through the radiator, a slipping fan clutch, an inoperative electric cooling fan, a collapsed lower radiator hose, an eroded or loose water pump impeller, or even a defective radiator cap.

Loss of coolant because of a coolant leak is probably the most common cause of engine overheating. Possible leak points include hoses, the radiator, heater core, water pump, thermostat housing, head gasket, freeze plugs, automatic transmission oil cooler, cylinder head(s) and block.

Make a careful visual inspection of the entire cooling system, and then PRESSURE TEST the cooling system and radiator cap - with the engine cold. A pressure test will reveal internal leaks such as seepage past the head gasket as well as cracks in the head or block. A good system should hold 12 to 15 psi for 15 minutes or more with no loss in pressure. If it leaks pressure, there is an internal coolant leak (most likely a bad head gasket but possibly also a cracked cylinder or engine block).

It is important to pressure test the radiator cap, too, because a weak cap (or one with too low a pressure rating.

You have a problem with the electric cooling fan, the fan should turn on when the temperature rises above a certain temperature.
1. Locate the cooling fan temperature switch on the thermostat housing. The thermostat housing is behind the distributor on the lower radiator hose.

2. Unplug the connector on the temperature switch and jump the Green wire to the Black ground wire and verify the cooling fan runs. If the fan does not operate, check the relay, fuses and wire harness.