Which fallacy is this?

In terms of "proof" this would simply be a non-sequitur .

In terms of "suggests" or "evidence", the fact that we went once and never been back could be more evidence for the fact that we never went then evidence for the fact that we did go. Basically, this comes down to the evaluation of evidence. I do believe that we have been to the moon and I don't question it to the point where I felt the need to research the topic (basically, I really don't care), but I were to investigate this issue specifically, I would look at humanity's other similar explorations to see if it is common that we go somewhere then never go back rather than stay there and continue to explore. Then, of course, look into the official reason(s) why we never went back (funding, risk, nothing more to learn, etc.) and perhaps interview experts on the issue to see if that makes sense.

Bottom line, to suggest since we never went back to the moon is "proof" or even "probably" means we never went is a non-sequitur . To suggest that this evidence on the side of us not going isn't unreasonable , but I couldn't say it is reasonable either without investigation or more information.