It's a second hand bicycle that I've owned for a short time, but the problems seem to have started after a crash. The headset started to rattle, and I didn't have a tool to tighten it, so I would just do it with my fingers every 30 minutes of riding. Now that I have a tool, when tightening the locknut, it "overturns", i.e. you can't tighten it.
I'm guessing that it's either the threads on the locknut that are worn-out or the threaded tube on which the locknut screws on. I'm not sure how to identify which though. If it's the threaded tube, is that easy to replace, or do you have to change the whole fork?