The Storm Sorcery sorcerer subclass, from Xanathar's Guide to Everything, has the Tempestous Magic feature:

Tempestous Magic

Starting at 1st level, you can use a bonus action on your turn to cause whirling gusts of elemental air to briefly surround you, immediately before or after you cast a spell of 1st level or higher. Doing so allows you to fly up to 10 feet without provoking opportunity attacks.

What happens if a storm sorcerer with 1 hp left uses Tempestuous Magic to fly 10 feet before casting Chaos Bolt, but gets hit by a crossbow bolt from a thug who had readied an action to "shoot the sorcerer if he moves", which knocks him unconscious?

Does the sorcerer:

• Still cast the spell, even though he is unconscious?
• Not cast the spell, and preserve the spell slot?
• Not cast the spell, but still expend the spell slot?

The Storm Sorcery sorcerer subclass, from Xanathar's Guide to Everything, has the Tempestous Magic feature:

Tempestous Magic

Starting at 1st level, you can use a bonus action on your turn to cause whirling gusts of elemental air to briefly surround you, immediately before or after you cast a spell of 1st level or higher. Doing so allows you to fly up to 10 feet without provoking opportunity attacks.

What happens if a storm sorcerer with 1 hp left uses Tempestuous Magic to fly 10 feet before casting Chaos Bolt, but gets hit by a crossbow bolt from a thug who had readied an action to "shoot the sorcerer if he moves", which knocks him unconscious?

Does the sorcerer:

• Still cast the spell, even though he is unconscious?
• Not cast the spell, and preserve the spell slot?
• Not cast the spell, but still expend the spell slot?