If your task manager is disabled and it appears greyed out then you can follow these steps to enable it again:
Click Start -> Run. Type in "regedit" (no quotes) and hit Enter.
Search for HKEY_CURRENT_USER -> Software \ Microsoft \ Windows \ CurrentVersion \ Policies \ System.
Look for: DisableTaskMgr. Click on REG_DWORD. Value: 1=Enable this key (disables TaskManager); Value: 0=Disable (actually enables TaskManager)
Reboot your machine.
If the task manager is again disabled after you restart your pc you will probably need to scan your pc with an antivirus. In most case malwarebytes will work. Click here to download malwarebytes and do a full scan..