Need to reinstall the game??

First you can check the game files for errors in Steam via game properties -> local files (might be named differently as I'm translating). Corrupt files will be updated automatically.
Your GTA online characters are saved on Rockstar servers. So as long as you know your SC credentials, you don't loose your progress in GTA Online. When you reinstall your game, you might loose your single player progress, depending on what you have picked when installing the game initially to save your game locally or on Rockstar servers.
But you should be able to create a backup of your local savegame under C:\Users\user\Documents\Rockstar Games