Replaced battery now won't start

We replaced a bad battery and now the van won't start. We had it put on a computer and checked alternator and starter and they are fine. It was suggested to us that maybe it needs a security code because battery was removed. We bought used and have no security code info. We were told to put key in and turn it for so many minutes to reset but still will not start. My husband is paralyzed and can no longer fix it and we desperately need the van. Thank you