From what I can gather from various sources (most detailed one here: https://www.businessinsider.com/elon-musk-admits-thwarting-ukraine-attack-not-activating-starlink-satellites-2023-9?op=1)

1. Elon's biographer Isaacson says he turned it off as the Ukrainian ships were approaching Sevastopol, following a call with the Russian ambassador who he told about it(!!?), resulting in a nuclear threat. Ukrainians begged him to turn it back on and he refuses.
2. Elon has denied the first part, saying it was never turned on, only acknowledging the call to turn it on and his refusal to do so.

I'm not sure which to believe. Isaacson's account is detailed and I doubt it's made up, but perhaps he misunderstood something. At the same time, turning it off seems to require Elon to have foreknowledge of the attack which seems unlikely, though plausible.