To me, the obvious things are battery (already mentioned) and the battery cables. Remove the cables from the battery, solenoid, starter and ground. Remove the battery, check the electrolyte levels - top as needed. Charge it with a 2 amp charger overnight. It should be 12.8v - 13.4v at that time. If not replace. Closely inspect the cables for creeping green death (copper corrosion). Replace if you find any. Clean the metal on each area down to shiny metal - coat with dielectric grease. Replace the battery, cables and it should start.
jb
jb