"I don't know why this is happening."
The problem is very simple: you do not extend x in the same way that you extend y and z.
For both y and z you extend them using the indexing i+1 before trying to access that new index position. No problems with them!
However, with x you try and access x(i) on the next loop iteration without making any attempt to define that index position first. In other words, you never make x larger. So on the 2nd loop iteration, your code tries to access x(2) which does not exist!
"I think I have used this procedure for the following loop many times, but now it isn't working:"
Nope, it never worked trying to access an index position that does not exist. You must have changed something.
Perhaps the last line should be this?:
x(i+1) = x(i) + h;
