I did this too, but I quickly managed to fix it.
Here's how you do:
1. Rightclick on any of your toolbars, preferably the one you want the window manager in
2. Select 'Add to Panel...'
3. Scroll all the way down until you see 'Window list', with a description of 'Switch between open windows using buttons'
4. Select it and click on 'Add'. You should now have the window manager back.