Nissan Repair: 2002 Frontier Transmission?, correct weight, grease

Question
My son just changed the oil in his 2002 Frontier.  Now he said when he tried to shift, the transmission would not engage.  Any thoughts on what may have happened?

Answer
SR,

If it is an automatic the fluid is probably low or the filter was installed incorrectly and is sucking air.  Check the fluid and if it is good then pull the pan and inspect the filter.  If it is a manual be sure the correct weight gear grease was used.