Turn key all dash light work but will not start

I was having overheating problems so I took my car to the mechanic, not my usual mechanic because his shop has been closed for some reason. Took it on Monday and this mechanic said he could not find anything wrong, picked it up later that afternoon and drove quite a while with no incidents. On my way home from probably a 20 mile trip out, the heat started to go up and track went of to I pulled over and waited a while. It happened a second time so I pulled over and noticed that when I was adding fluid to the coolant tank the water seemed to be coming out from around that area but not the radiator. Took it to the same mechanic the next morning, Tuesday, and he connected the scanner and turned the car on and off a few times and told me he believed the problem were the cooling fans. I left the car with him and he called about an hour later asking if I ever had a problem starting the car, grant it this is and older car with a lot of miles but it always started on the first crank. I went down and when I tried to start the car all dash lights would work as normal on the start position but when I tried to crank the engine, nothing, all dash lights would go off, including security light but not even a click sound when I tried to start the car. This mechanic told me it had to be the module because he checked everything else and said there was nothing else it could be. Now i'm checking online noticing it could be a host of problems. The car was starting just fine even for the mechanic before I left the car with him. Could the computer module go just like that? What else could it be, he's had my vehicle for 4 days now because they were delivering the wrong module, now I just spoke to him and he says he has the correct "computer" part and hes programming it.
Any of this make sense, please advise.