Make sure to check your personal information, like your name & address. Make sure the information matches what the CRA has for you. If you are not sure what they have on file for you, you can call and confirm it with them. 

If you are in Canada or the United States, you can call the CRA at 1-800-959-8281. If you are elsewhere, you can call 613-940-8495. This page has more ways to contact the CRA: Contact the Canada Revenue Agency