Why wont my Pontiac Montana '01 wont go into any gear, but will start? Also noticed that the security light started flashing when that problem started.

with the engin off, have some one look under the hood to see if the lever on the transmission is moving when you shift it thru it's gears, or if it's on a bit of a hill, take it out of park and release the brake to see if it will roll a bit. the shift cable might have come disconnected.
is there a check engin light on?
did you notice any abnormalities befor this problem happened?

Thank you for your advice. I will do what you have suggested. I tried to go out in the blizzard and ended up getting stuck in the snow. A thought a plow would be coming soon and I could get my van out, so I left it there with the hazzard lights on. Needless to say, I was not plowed out as soon as I thought and the battery died; when the plow did finally come a tow truck was right behind. Other than that, nothing out of the ordinary.

let us know what you find.