In some situations one or multiple tasks will show the FAILED status in the Task History and the error stage will display the affected task.

If the Error stage is assigned to you, you can simply select the ERROR failed task, and then hit the Complete task - Retry buttons from the Stages section.
If the Error stage is assigned to a different user then you first need to assign it to your user:

1. Open the affected project 
2. Click on Stages and select the ERROR failed task 
3. Click on Reassign Task and select your user
4. Go to Inbox -> New -> Select the ERROR failed task and Accept it 
5. Once the ERROR failed task is switched to the Active tab -> Select it -> Retry

The root cause may differ from project to project and it can be related to the project configuration, resources not being accessible, file issues etc. The above presented example shows the issue with the Machine Translation task FAILED.