Don't have one, so maybe sort of "general" info...
Try a different 18650, and charge outside the
mod - i.e. don't use the USB charge port. If the battery is too low, it could take a long time for it to charge (if at all!), especially with on-board charging. With many regulated
mods, there's a small drain on the battery even when no display is lit. Over time, that can yield a "dead" battery.
Make sure the battery polarity is correct for the
mod. If in backwards, it won't power up. (And some don't have reverse polarity protection, putting it in backwards will result in failure. Yours says it does, so just won't power up if backwards.)
Good luck!