Playing on level for many hours (I think 5-6hrs worth), went to sleep and computer crashed.

Level: https://www.warzone.com/SinglePlayer?Level=1436918

Normally if something happens, I come back and it asks me if I want to resume or start a new game.
This time it just assumes I want to start a new game. I hit refresh (F5) and it just keeps restarting the game without a dialog box.

Am I out of luck with resuming that game?

If you check https://www.warzone.com/singleplayer/savedgames you might be able to find it if youre lucky.

But given that your PC crashed, I think there is a reasonable chance you won't be able to recover the level :(

It shows up as level 1. I guess I am out of luck.

Thank you for the link to try

Do you know if there is a manual way to save future games?

No, single player levels are stored in the local storage of your app/browser.

I suppose you could extract it from there and manually re-add any missing keys, but that sounds a it too hacky to actually suggest 😅