You're not entitled to a cape. Its a gift for supporting the mod, I hope you know that.
Anyways the only way to get your cape back if you changed your username without doing any research first, is to login using the email when you bought the cape here: https://optifine.net/login If you forgot your password, click that link. If someone else bought the cape for you then contact them. If all else fails, you must donate again to get the cape. Supporting a mod you use all the time is a great way to thank the mod developer for his time and effort!