had to a hard shut down to escape from the online scammer attempt

While on my apple computer-I had an encounter with video and noise that my apple computer had a security issue and was frozen by apple support. Instructed to call a Apple support number to walk thru my computer. This was obvious scam-so I did a hard shut down though it screamed not to. How do I know my computer is safe? Used MalwareBytes to run a scan--it found nothing.