Layoff notice, what does the 60 days mean? after 3 years as a customer-service rep

My employer announced that the site will close and gave us written notice. It has been weighing on me all week. I do not know if I should start looking now or wait for severance news. Has anyone been through this?

Write down key dates and keep copies of the notice. Mine helped when a question came up later.

I stayed for the retention bonus and looked for work on the side. It cushioned the gap.