Why is it not turning

First thing I would check is the battery - if you have the old NiCds, nearly all of them died years ago - I have an XR 20V with the hammer drill option (well, two from Lowes sales when you buy batteries) and mine were DOA with my DC970 I run with the modern drills - one was dead years ago and hinging on faith (it was gone within no time when I was given the drill and batteries!), the other went out shortly after I inherited my DC970 for good in a few months (previous owner didn't want to convert it - they wanted a modern Brushless). Most tools can be converted with the DCA1820 and a base 2 battery drill bundle for the price of batteries on their own or even the kit. It's a partial waste because you don't need the drill but it's the cheapest source for (albeit, weak and one will die first) the 2Ah batteries and needed charger on sale when it's at the $100 mark - otherwise it's not much more and tends to be in spitting distance of the battery conversion bundle for these old drills with the same weak 1.5-2Ah packs.

If you want the 3-4Ah packs, you buy those piecemeal -- 5Ah 18650 (not PowerStack) is fine and what DeWalt recommends for the DC970 at the max (but they're also all out of warranty) so given the warranty implications I would play it safe and charge MORE often with 4Ah.

This is brushed but it has the precious charger+1.5Ah batteries: DCD771C2

PowerStack packs do not fit the adapter by design but work on the 20V tools so they gatekeep those.

If the batteries do not work, it's likely the switch or motor. DeWalt stopped selling parts for these years ago, sadly. That said, 95% of them are bad batteries from the early 2000s.

NOTE: Never leave the battery in the DCA1820 - it drops to 18V with resistors which slowly drains these packs. Pull it but leave the adapter in. It's not a permanent install and can be moved, but I leave mine in so I know I'm ready.