Poster session without a poster
Unfortunately my answer comes too late for the OP specifically. 

First, you should contact the session organizers to explain your situation. This is important because you do not want to be regarded as a no-show by the organizers. Depending on the conference, it may have a negative impact being able to present at the same conference in the future.

Second, you would want to avoid wasting your opportunity to present at a poster session. As others have suggested, try to print a full size or even a partial size poster, or even just your major figures. Ultimately the point is for attendees to see your research, results, affiliation and to network, and any kind of information is better than no information. As long as its enough for someone to come up and talk to you, I'm sure you can fill out the rest of the details by your memory.