auto tranny problem need help asap

[nissan-fanatic]

Ok while driving I started hearing this humming like sound coming from the tranny and the gears wouldn't change like it suppose to. It took a bit longer than normal. so when i got to my wife's school to drop her off I checked my auto tranny oil and saw there was hardly any. So I wanted to go to O'reilly auto parts to get some tranny fluid but when I went to put it in reverse, IT WOULD NOT GET IN GEAR! Nor any other gears. It was like it was stuck in neutral. So I turned off the car and called a friend to find out if he knows wat it could be since he's a mechanic. He said it sounded like my torque converter could be out. So now I was stuck, broke, can't afford to tow it. But then i decided to turn the car back on after awhile and now it was able to get in gear and was able to make it home. Thank God. Wat can be the problem? I'm bout to try putting fluid in to see if that was it.