Possibilities to consider if function is not working

Question

If a function is not working for some reason there are three possibilities to consider
1. precondition
2. postcondition
3. something wrong with the return value

what are these conditions? explain with examples.

Answer 1

Sounds like homework... Show your own attempt.

Answer 2

https://www.programmingincpp.com/precondition-and-postcondition-of-a-function.html