Lucky today

[totta Crolla]

I think anyway.
Went to start the car and it did'nt start first time, second attempt it turned over and then made a horrible noise from the engine.
After a little investigation I discovered that the cam belt tensioner spring had come adrift and pierced the cambelt, it was trying to rotate with the belt
The belt was very loose and had jumped a couple of teeth, I expected that but I had not expected the small stud that the tensioner pivots on to have come out completely.
Anyone care to suggest a series of events for what happened here ?
Lucky ? Just a new belt and spring needed.