Dont know exactly, but if its like a pug, there is a small black "brick" inside the key that is loose. If yours is the same and the key unit was "open" then this small black brick may have fallen out. This brick ios the transponder that when the key is inserted into the ign switch, talks to and confirms that its the correct key, so allowing the car to start etc..
If it gone, then the car will never start as the immobiliser will not allow the car to start as it thinks you are a thief. Good eh!
I doubt very much the cam belt has gone as serieous noises would be coming from the engine, banging noises. It quite easy to check if the cam belt has broken but needs 2 people, one to operate the key and the other to look thru the oil filler cap hole looking at the cam shaft that will turn when the engine is cranked. This will only confirm that the cam belt has not broken, but may have skipped a tooth or 2, but its the first place to start to rule out a totally broken cam belt.
Hope its just the transponder which is small, probably 3mm x 3mm x 5mm, small and black if its similar to the pug type.
If some could post the parts exploded view of the key unit it will show the transponder hopefully.
good luck.