When you press Ctrl+Alt+Del that shold bring up your task manager. If you get an error message when trying this then it is very likely that your computer has been infected. Please run a complete virus and malware scan and see if that fixes it for you.

One more thing you can try is locate your taskmanager - navigate to C:\Windows\System32 and search for a file named taskmgr.exe, if it is there and gives you errors when double clikcing it then you should try the first step I suggested.

If you on the other hand get a message Task Manager has been disabled by your administrator then you can fix it by editing your registry.