We have identified an issue with the PC version of the game that can corrupt a player's character data, making the save file unusable and resetting all progress. A fix is in the works already and will be released as soon as possible. In the meantime, please follow the advice below:

To avoid losing your character progress, do not close the game during start-up before you have reached the main menu.

As an extra safeguard, you can make a backup of your save game by saving a copy of game.save in \...\Program Files (x86)\Steam\userdata\#user#\22350\remote\ before you begin play.

This issue does not affect the Xbox 360 or PlayStation 3 versions of the game.