If the halo was colourless and 22 degrees in radius (approx. thumb-tip to little finger tip with hand held at arm's length) then it was probably an ice halo caused by refraction and reflection from tiny atmospheric ice crystals.
If coloured, then it was a 'corona' caused by diffraction or scattering by cloud water droplets.
I'm not an expert though! I got this from:
http://www.atoptics.co.uk/droplets/cormoon.htm
and
http://www.atoptics.co.uk/halo/circmoon.htm
Fascinating stuff!