In addition to the above note that Malwarebytes Free can be installed, updated and run all in "Safe Mode with Networking" if necessary. (To keep it free do NOT accept the free trial offer).
With ransomware which this probably is the best escape is at first hint of trouble hard power off, because any mouse click or movement or keyboard click will activate it and basically you are doomed. The only solution is to format and reinstall the system as paying the ransom rarely works.
Then reboot into Safe Mode and hope that System Restore will work to take you back to before it happened.
Best support for this kind of thing can be found on the specialist malware removal forums such as BleepingComputer etc.