every time you create a save with "save as" using a new slot you are creating a backup of your game. A good practice is to rotate 3 slots, each for about 1week to 1month older than the next.
now all you need to do is load another slot (manual backup). You will of course have to replay the last few days again.
IF you have no backup at all: then you will need to start over. You can use cheats to speedy up your game (get items, money, conquer territory, find companions, etc) to the point you were earlier
TLDR: backups are life. Keep many, do it often.