If you have been banned, it was most likely because you violated the fair play rules listed here:
Please take note that even if you were playing with friends or family briefly this may have been enough for a rightful ban.
If you still wish to play RISK, your best bet is to create a new account and stick to the rules. Any such new account will be under additional surveillance for the first few months. Note that purchases will not carry across accounts under most circumstances.
If you are 100% percent sure you never violated the fair play rules and never cheated, you may open a support ticket and challenge the decision.
Please take note that false claims in such disputes can lead to a lifelong ban!