Transmission problem

I have a 2000 Ford Econoline van with approx 72,000 miles on it. It is a 6 cylinder engine. We bought the van used in March of 2007 and it ran great until this weekend. While traveling to visit friends, driving down the interstate at about 65 mph the van stopped its forward motion and sounded as if I had accidently knocked it into neutral. I checked to be sure it was in gear and unfortunately it was. We were only about 30 miles from where we were going and the van would drive in 2nd gear so we drove very slowly and have left it there for now. It goes into reverse, 1st and 2nd without any problem but according to the mechanic when it tries to shift into 3rd or 4th it goes into a "false" neutral. He has tried to check the computer but cannot get it to, in his terms, "fireup". Can you please help me with an idea of what could be wrong or how to get the computer to come on so we can be sure of the problem? He said there was no shavings in the pan when he dropped it and the fluid is not burnt smelling. Being the van was used it didn't have an owners manual.