When you are trying to open the task manager by CTRL+ALT+DEL then are you getting the following dialog box saying "Task Manager has been disabled by your administrator".
There may be several reasons for this to happen. 1. You use account that was blocked via the "Local Group Policy" or "Domain Group Policy". 2. Some registry settings block you from using "Task Manager". 3. Your system has been infected by a Trojan that is blocking you to use you Task manager.
The thrird reason is the most dangerous one and the only solution for that is to update your Anti Virus Program, Scan your system and remove the Trojan.
How to fix the other issues we will discuss it here :
The best thing regarding this that i will recommend you all is to go through the following Microsoft support document and follow the procedures mentioned there: Microsoft Support
Other thing that i found out is a toll to fix the registry for the same. You can get the tool from here -> Registry Tool