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 and colluded against strangers more than two times this might 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, but purchases are transferable.
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!