Plant closing and notice period questions

I received a formal notice about a mass layoff at my workplace. There are 13 people counting on me, so I am trying to be careful. I do not know if I should start looking now or wait for severance news. Should I stay for the bonus?

Thanks for writing this out. I went through something close to it 8 months ago, and it settled down once I kept a simple log of every call and form.