Can't play The Following - it goes to the regular campaign in the Slums

Try receiving an invite from a co-op friend and join them in the Following, please.

If it does no work, then your save files have become glitched/hacked and are now corrupted, which can result in  further glitches and/or lost content.

There is no way to fix corrupted save data, but if there is one more save file with some progress present in your game, you can try the steps below:

In the game's menu, go to Play Campaign, choose a NEW GAME save slot, press the Advanced key indicated below the chart and then choose a chapter from the story where you would like to continue the game from.

Doing this will allow you to start from the beginning of the last mission you have unlocked. It will also give you Skill Points for each Skill Tree, which you can spend however you wish. The amount of skill points you have is determined by how far you have gone into the story. You will also have a random pack of weapons, throwables and consumables appropriate for selected game progress. Additionally, all of the unique collectible items which you could have obtained by that particular moment in the game will be present in the environment for collecting.

Unfortunately, we cannot restore the character to an exact state from before the  issue.