Restore the original task manager after replacing it with the Sysinternals process explorer

Delete the subkey Debugger of the registry key HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\taskmgr.exe.

In cmd, you can accomplish this by running (as admin): reg delete "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\taskmgr.exe" /v Debugger.

Don't delete the whole key, just delete the Debugger string value.