Cause: Server crash. Solution: Re-exit in online.

1. First, open Steam and before the game starts, click on Related Groups in the left link bar on the left Road 2.

2. Select Left 4 Dead 2 and double click your mouse to enter the group.

3. Then wait for Steam to open the game and click the Finish Campaign option in the left column.

4. Click the mouse to complete the campaign under the online play option.

5. Then select the room you want to enter and double click on it to enter the online mode.