Huh, that is super bizarre. What this indicates is that you somehow set your score on level 1 to a negative number, making the game think it isn't unlocked. I'd try to either reset your browser cookies somehow or to open the game in an incognito tab / another profile or something. Apologies! Will definitely include a "reset save data" button in my next jam submission.
Unfortunately, as long as you're hitting return to properly move through the menu, I can't tell you why you were unable to move. Hopefully if you open another window it won't be the case.