It is possible that your data may become corrupted. This could freeze your task list, and in the worst case it could freeze or crash Outlook.
If this happens, there are three possible solutions:
First, you can try cleaning your data. To do this, restart Outlook, and press the Clean data button in the Tasklist tab on the Outlook ribbon:
This will run a utility that will attempt to detect and fix problems with your data. This may result in promoting some child tasks to root tasks if they have become disconnected from their parent, so you should examine your task list, and drag any disconnected root tasks back to their parent.
If this doesn't work, try to restore your data from a backup that worked. This is explained here.
As a last resort, restore your data manually:
•Locate a backup file that worked. To do this, open Windows Explorer, and navigate to C:\Users\[your name]\AppData\Roaming\Tasklist\Backup. (The AppData folder is hidden by default; to see it, go to View in Windows Explorer, and check Hidden items.) Here you should see your backup folders, designated by the date and time of the backup. Open a folder from a date that worked. You should see a file called Tasklistdb.db. This contains your task list. You should also see a folder for emails, and a folder for notes. These contain the backed-up emails and notes that you saved with your tasks.
•Locate the data folder that Tasklist is using. This is in C:\Users\[your name]\AppData\Roaming\Tasklist\Data.
•Replace the tasklistdb.db file and the emails and notes folders in the Data folder with those from the backup folder.
If all else fails, uninstall Tasklist:
•Close Outlook. If it won't close in the normal way, close it from the WIndows Task Manager (press Crl-Alt-Delete), or reboot your computer.
•Click on the Start button (bottom left), open the Control Panel, and search for "Uninstall a program."
•Find Tasklist in the list of installed programs, and double-click on it to uninstall it.