Welcome to the forum!
Are you using the "Triangle In" shape option on purpose? When you use that particular shape option, the Powerstroke LPE is automatically applied to the path. For a reason which I don't understand, powerstroke doesn't work on a closed path. (although I think I heard that the next version will fix that....it seems like I heard that, but can't remember where)
Also, testing on my own canvas, I'm finding it's behaving strangely with corner/cusp nodes..... This has to be a known problem -- it's too blatant!
Well anyway, for your problem, if you didn't use the powerstroke on purpose, just choose None in the Shape dropdown menu, on the control bar. Or if you don't want to draw the path again, then select it, and do Path menu > Remove Path Effect (bottom of the menu).
If you are using it on purpose, don't close the path. If you need the path to at least look like it's closed, we can help with various tricks.
Oh! Or if I'm correct that it's going to be fixed in the next version, it might actually already be fixed, which means you could install the development version.
Let me do a little research, and maybe I can find out for sure.