First of all ensure your server is up and running as if the server has crashed, this will prevent TaskCantre from being able to run.
Assuming the server is working, you will then need to check TaskCentre itself to see if you have a stuck task. You can identify this by the time stamp on the tasks currently running. Right click on the task and select Terminate Task to end it.
If you have no stuck tasks, the most common cause for tasks not running, is that the services have stopped running. To fix it, please see our article Orbis: How to restart the TaskCentre services.
If the services are running, or restarting them does not fix your issue, your licence may have expired. You will see a message when logging in to TaskCentre advise you of this.
Please see our article Orbis: Our licence has expired and tasks are no longer running for details on how to resolve it.