given you failed to disclose your US visa refusal, it's not really surprising your application was rejected. Did they simply refuse your application or have you received a Procedural Fairness Letter with it. If you haven't received a PFL, consider yourself lucky, as they would have banned you from Canada for 5years.
If it's a refusal, you can re-apply, but make sure you include your US, and now your Canadian visa refusals. That's about the only choice you have.