Failed smog certificate test due to OBD catalytic monitor "not ready."

What is your question?

I have received advice from about a dozen people. Each has a different solution. Basically, I am told to complete a "drive cycle" that is highly involved and looks like it will be very difficult to perform. If am going to do that and if that doesn't do it, my mechanic said he will do it for me for $150 and not return the car to me until it is ready to pass. I suspect I am going to be handing the car over to my mechanic in a day or so. But I want to know, how did this happen? How did my car's computer (the OBD) suddenly become "not ready" with regard to the catalytic monitor? Because I don't want this to happen again. If it was something I did wrong that I can prevent doing in the future, I want to know about it.