Apologies if this has been said before, but could be an issue with the craft having a landed state?
I had a craft with the orbit line not visible, and while I was cleaning up my orbits by removing some debris, I noticed a craft that I didn't know about showing as "landed" on kerbin. I hit "recover" and then my bugged craft dissapeared. I reloaded an earlier save, saved again and saw that the craft had the "landed state", despite having an SMA of over 1 million.
Manually editing the save from `"Situation": "Landed"` to `"Situation": "Orbiting"` (and then loading that save) brings the orbit line back, so it seems that is a fix for now that doesn't involve re-opening the entire game. You can use any text editor to edit the save file, since it's just a JSON file. I use VS Code to edit the file because I already have it installed, but I'd reccomend Notepad++, but you can of course use regular Notepad if you want.
I've no idea why the craft is being given the landed state, but from the other posts here it seems like there's several potential causes.
Hopefully this helps those of us with slow computers who'd have to wait 20 minutes to relaunch the game
