Just so we do this right the next time... DDOS... Distributed Denial of Service.
DOS is an operating system.
But you knew that...
No, actually, I skipped the DOS generation. Was one of the last VAX/VMS programmers in my neighborhood. Jumped straight from VAX/VMS to Windows. Never had any interest whatsoever in learning about P/Cs or DOS.
Both are legit terms denial of service attack and distributed denial of service attack:
Most likely it was an DDoS attack though.