Short answer: Yes, according to the official rules you could be denied access at the border. You can avoid this by submitting the green form (along with additional documentation) after the fact.
Most I-94s these days are recorded electronically. Before sending in your evidence as described below, check your electronic records at https://i94.cbp.dhs.gov; if your departure was recorded correctly, you don't need to do anything.
It sounds like you participated in the Visa Waiver Program and the green form was an I-94W.
According to the US Government,
If you departed by land, private vessel or private plane, you will need to take steps to correct the record. If you do not validate your timely departure from the United States, or, if you cannot reasonably prove you departed within the time frame given to you when you entered, the next time you apply for admission to the U.S., Customs and Border Protection (CBP) may conclude you remained in the U.S. beyond your authorized stay. If this happens, your visa may be subject to cancellation or you may be returned immediately to your foreign point of origin.
[Emphasis is mine.]
If you still have the green form, you can remedy the situation by mailing it to:
Coleman Data Solutions
Box 7965
Akron, OH 44306
Attn: NIDPS (I-94)
USA
if using the U.S. Postal Service, or
Coleman Data Solutions
3043 Sanitarium Road, Suite 2
Akron, OH 44312
Attn: NIDPS (I-94)
USA
if using FedEx or UPS.
You should also include a copy of the boarding pass you used to exit Canada. You should also provide a letter explaining your situation and why the form was not originally collected. Proof that you lived elsewhere after the 90 day period of the visa waiver (e.g., rent checks, pay stubs, receipts, &c., from after you got home) would also be helpful.
Note that the visa waiver program also counts time you spent in Canada and Mexico. Therefore, you will need to show that your total time spent in Canada and the USA was no longer than 90 days.
The original US government FAQ page does seem to suggest that it may be enough to simply bring this documentation with you the next time you enter the US. Regardless of whether or not you mail it to the government in advance, you should definitely bring a copy of all of the documentation with you on re-entry. Also, if you are really worried, it seems like you will have no problems if you apply for and receive a formal tourist visa before re-entering the US, but that process will also likely require you to provide the same documentation.
