Losing site definitions. Please help.

Hi to all. I opened Dreamweaver today the same as any other day and was
suprised that I had no site definition for my site! I re-entered all the
settings, published my updates and closed Dreamweaver. Came back later and
again had no site defined! Reset, closed Dreamweaver, re-opened and gone
again. Has anyone ever encountered this? Using MX and am fairly new to it.
What am I doing wrong? Some setting I need to change? Thanks in advance. Joe