My watch was misplaced for about 6 months. Battery is totally flat and it now won’t charge up. Do I need a new battery?

When you place the watch on a known working charger:

1. Have you left it on the charger overnight?
2. Does anything show on the screen?
3. Does the watch get warm/hot?

Have you tried everything here:

If your Apple Watch won't charge or it won't turn on - Apple Support

Possibly, there's no way to know for sure unless you bring it to a technician to check.