In a properly working market, at the point the power grid fails due to lack of supply, the spot price should be infinitely high.

Given that, no sane supplier would refuse to sell given they would be making unlimited profit. Hey, I'd be connecting up my windup flashlight if I got paid $999,999,999 per kilowatt hour.

The issue must be a bad market. Either people aren't paying for what they use, there isn't sufficient transmission capacity to have a sufficiently large market to avoid local monopolies, there are price caps or floors, the market isn't fast enough (no automated trading), or new entrants are limited from participating.

In a correctly working market, electricity never fails because all people using electricity at the time of the failure would be paying an infinitely high price, and therefore be bankrupt.