I'm amateur and got this stationary bike. The crank arm, I think that's what it's called, fell off. I can put the bolt back (the one inside the red circle) and turn manually but can't tighten it. There was a cap on top of it. What tool do it need here?
That nut was put in the wrong way around.
This is what the nut looks like:
The hex end is what you should tighten with a approriate socket.
This is what it should look like when installed correctly:
If it ever get's super tight as it is now, to get it off, the easiest way is probably to cut it with a tremmel and chisel it out.