My first step, would be to use the Eraser on an 'Old School' wooden pencil to 'burnish'/polish the battery contacts inside the light. Pay attention and do a thorough and carefully gentle job. The goal is to remove any surface oxidation on the battery contacts that probably formed while the light 'Sat Around Unused'.
Carefully blow/brush out any Eraser 'crumbs' from the battery contacts and compartment.
You can finish up, by wiping down with 90% or better Isopropyl alcohol if desired. Avoid the temptation of using the cheaper 'Rubbing Alcohol' (70% or so, Isopropyl Alcohol) as it will contaminate the contact surfaces with mineral oil.



/