1. Go to C:/Program Files/Microsoft Games/Gears of War/WarGame/Checkpoints

2. Depending on whatever chapter your on for example: if at chapter16, find that chapter and chapter 0

3. Now back up chapter0 and chapter16

4. After backing up chapter0 delete it

5. Rename chapter16 to chapter0

6. When you run the game dont 'start new campaign/ continue campaign'.
Instead select the first act/checkpoint manually and this should put you where you were supposed to be
The previous acts will not be unlocked but it at least lets you continue from where you left off.