Did you try scanning with Malwarebytes Anti-malware scanner and a good antivirus program as suggested by strollin?
If SFC /scannow has not worked for you, you may try the following tips.
A. Try to go to a restore point when the taskmanager was working.
or
B. Follow this procedure
1. Download sysinternals suite from microsoft technet and unpack.
2. Find and run procexp.exe.
3. Click in Options menu on Replace Task Manager and than Restore Task Manager.
4. Click Ctrl+Alt+Del or on taskbar Run Taskmanager.
or
C.Click Start, then type regedit.exe into the Search box and click OK.
Now navigate to this key: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System
On the right you will see the value "DisableTaskMgr".
Double-click it, then replace the "1" with a "0".
If you do not have this key in the above location then simply close regedit program.
After these procedures check to see the taskmanager is working or not.