Check the validity of your files in steam (if you use that)
Check the game files yourself and make sure the important bits are there (save game, game data folder)
You may be signed out of steam
- You may have internet problems
I remember steam uploading always being annoying, it takes me ages to get connected, so I never use it
Kerbol
The Sun (or Sol) is Earth's sun, so to avoid confusion Kerbin's star should be called Kerbol. Its called this on the wikis and I think in game, but I'm not sure about that.
IF you use steam, then just connect through bluetooth and then steam will do everything for you. If you don't use steam then I don't know, but you could try connecting to bluetooth