Employees often know, or had a sense that, their termination was coming. However, most employees in this position do not start looking for work as soon as they felt their job was in jeopardy. This is a mistake.
Voluntarily seeking a change your job can be quite daunting. Especially, if you are comfortable where you are. However, starting a new job search as soon as you sense that your current position might be in jeopardy is critical. First, sending out resumes and interviewing for new opportunities is a chance to get a sense of your value as an employee. You may just find out you could be doing so much better both in title and compensation had you just looked around.
Second, whether you find something you would consider or not, going ion interviews and being made offers will give you a sense of empowerment that will allow you to better cope with the stress of your current position. Indeed, many of my clients wind up with a windfall because they began their job search at the first sign of trouble. They were able to obtain a new position shortly after being let go and therefore bank all of their severance instead of having to live off of it. That is the ultimate severance coup.