ENABLE TASKMANAGER

If you have any viruses, then you may notice that they affect to the task manager and disable it
This is because if we know that there is a virus active in our background there’ll be a chance to stop it using task manager.
Disable task manager doing by viruses editing the computer registry.
Here how you can fix and get back the Task manager again.

"Method 1":Click Start >> Run >> and type the code below and press Enter.
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Pol icies\System /v DisableTaskMgr /t REG

"Method 2":Click Start>> Run>> and type Regedit.exe (open registry editor)
Navigate to the HKEY_CURRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Policies\ System .In the right-pane, delete the value named DisableTaskMgrClose Registry editor. You may want to restart you computer for changes take effect.

"Method 3":Click Start >> Run >> type gpedit.msc (Open Group policy editor)
Navigate to:User Configuration / Administrative Templates / System / Ctrl+Alt+Delete Options / Remove Task Manager .Double click the Remove Task Manager option.Change the policy to Not Configured and click ok