Task manager does not open or is disabled.

Open Notepad.

Copy-paste the following-

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System]
"DisableTaskMgr"=dword:00000000

and save the file with a .reg extension.

Exit and double-click the file.

Note that if your admin has done this you cannot modify the registry since you are using a limited account. If you are the admin yourself, probably some trojan or DOS tool has done this. Run a virus check if the problem persists even after running the reg file.