This is the main issue when facing viruses, the task manager is always disabled because you can kill the process and maybe be able to fix the problem. Sometimes it works, sometimes it doesn’t, you just need to try for yourself. Anyway, there is a simple and effective way to enable it again.
Open the notepad and type the following:
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD /d /0 /f
Then, save that with any name you want, and make sure you save it as .bat.
Then just double click it whenever you need it and you’re done. Note that this doesn’t have a 100% effectiveness since not all viruses attack the same way and not all computers are the same.