If your a tasks are not visible, and adding new ones don't appear, the issue is usually due to one of the xml files becoming corrupted under C:\Windows\System32\Tasks\Power Update. Try removing them (or moving them to a different folder) one by one until the tasks appear again (reopen Power Update after moving one of the files).
If the above doesn't resolve the issue, perform the same process with the tasks in the registry (remove Power Update related tasks one by one):
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\Current\Version\Schedule\TaskCache\Tree\Power Update
Also make sure the logged in windows user or the Everyone group has Full File System access to the C:\Windows\System32\Tasks\Power Update and C:\Users\[USER]\AppData\Local\Power Update folders.
There's also an open source tool to automatically try to repair the tasks based on the steps above: