Other institutions, retail whose brokers didn’t fail. Most activity on the markets in terms of volume would generally be, broadly, institution v institution anyway.
I’m not saying it makes it right, but retail is probably better off sitting tight for now. If anything, pros and institutions already had those sell orders ready to fire last night and would get in ahead of retail on market open, at least that’s my guess. So I don’t know what good it would have done me to be at the end of the line to sell my retail stuff.
OTOH, if you desire to go shopping for bargains, yeah, not being able to trade sucks.
It's an interesting topic. I'm sure this is just excess server load and nothing nefarious but could the SEC mandate an SLA? Require disclosure of their DataDog dashboard?