Chemotherapy and radiotherapy affect rapidly dividing cells such as cancer cells. This is why it is used to treat types of ovarian cancer.
Unfortunately, both chemotherapy and radiotherapy can also damage the rapidly dividing immature eggs in the ovaries, and this can be irreversible. However, this depends on the drug used, the course of treatment and the dose.
The general rule is, the younger you are (under 30) the more likely your ovaries are to recover. Whereas women over 40 are more likely to experience an early menopause because of the naturally reduced fertility at this age.
Not all chemotherapy and radiotherapy causes ovarian failure though, and your individual treatment choices can be discussed with your oncologist or GP. It does not mean you can stop contraception either.