Right-click Start, click Run (WinKey + R)

1. 
   Right-click Start, click Run (WinKey + R)
   
2. 
   Type Regedit.exe and press ENTER
   
3. 
   Go to the following registry key:
   

Look what the registry value named Debugger is pointing to. This tells you which program has hijacked or replaced Taskmgr.exe. In case of obscure file names appearing as the debugger, it could be a malware. Delete the value and run a thorough scan using Malwarebytes Antimalware or any other reputed malware and virus scanner.

4. 
   Right-click the taskmgr.exe key, and choose Delete.
   
   
5. 
   Exit the Registry Editor.