2002 Honda Civic Coolant

I was recently devistated when I came home from work parked my car with no fault, and then half an hour later tried starting it. To no sucess. It turned over and started for a second and stalled out.I thought it was the battery, but I just replaced it 6months prior. I could start my car and when I did it stalled. Took it to a dealership where they told me I had a faulty head gaskett, and my engine overheatted and needed a new engine $3000. How can I have no apparent warning signs? I did regular oil changes, maintence? Im devistated. Took it for a second opinion and they said it was my temperature switch. Replaced it. Ran fine for a week, but im completely parnoid to drive it. I checked my engine, my coolant ect. And I noticed that my coolant is low. Close to min. And well, I know for a fact that it was full when I left the garage. Although I dont see any physical evidence under my car that my coolant is leaking.I know its low. My engine didnt start I drove it from work 20mins and when I tried to start it it just kept stalling out. No smoke, no "dash lights", just no warning? Soo confused.