Most of us experience this sooner or later, but if you experience it really close to where your machine was delivered, here is the two top reasons why your laser tube died.
1: Overpowering the tube
While the tubes are called 40w, they cannot really handle 40w and has a effective output of about 30-35 watts depending on tube quality.
This mostly happens on the digital versions of the K40, where the milliamp. output isnt visible, only a percentage of the power.
Theese machines cannot handle 100%, they cannot handle 90%, you will overpower your tube at 90%, thus lowering the life span drastically.
If you have a digital version, i suggest installing a 30mA amp meter.
Keep your tube below 18mA at all times to be safe. You can go up to 20-25 but the gain is very little and you are strangling your tube for every second doing it.
2: Overheating the tube
This is pretty common, not having a clue about the water temperature in your tank and just running the machine.
The laser tube works the best at 15-20c water temp, anything above will degrade your tube.
If you run at 23c your tube will not die tomorrow, but you may lower the expected life time by a couple of weeks or month. So for some its ok to run at 23c as they know they need a tube change later on.
There is also a link between using wrong coolant to unexpected tube malfunctions.
Read more about that in the watercooling article.
But if you want to use your tube the most, keep temperature around 20c.
Whats happening when the temperature is too high is the regenerative gases has harder to regenerate inside the tube wich lowers the power output with time.
There is a deeper scientific explanation, but i keep it simple and understandable for now ;)