Overheating and ignition problems 1995 accord

I have a 95 accord. I got home one evening and couldn't get the car to turn off. I turned the key to the 'Off' position, took the key out, and the engine was still running. My power locks were not working either. It ran for about 2 hours, my fans stopped working and my temperature gauge went all they way up. My water tank tank pretty much dried up. Somehow I moved my key around again- like I was starting the car again, and this time it turned off. My mechanic claims everything in the engine is fine. He says I need to replace my ingition switch (where the key goes). ANyone agree with this? Anything else that could have caused it? Earlier in the day they'd connected the cooling fan directly and taken out the thermostat so that the fan is always on. But when the engine was running and the car was heating up, the fans were not working. I"m wondering if taking out the thermostat and connecting the fan directly had anything to do wiht my car not turning off.
ALso, my fans only come on when the temperature gauge goes past the 1/2 mark to about the 3/4 mark. ANy reason why this might be? I'd expect the fan to come on before the gauge gets to the halfway mark. I checked the sensor and thermostat butsill didn't work. Plleaaassseee Helllpppp!!!
I can turn on the car ok, no problems, but I cna't turn it off. With my keys out, the lights that initially come on when the car is turned on are staying on, even with the car off and keys out. The oil light, seat belt, etc.. Please helppppp

i would agree with the mechanic on the replaceing the ignition switch, as far as the fan comeing on at the half way mark, it's normal, if the fan comes on, then it's working. but your thermostat was take out you said....thats a problem, you should put that back in asap.