Plan A doesn’t always work out. That’s why it’s useful to have a plan B.
However, you can’t have a plan B for everything, all the time. There will be unexpected twists and turns for which you haven’t had the time to prepare a plan B.
When these occur, your best bet is to, to the extent possible, take your time. Although it doesn’t seem like it in the heat of the moment, a plan B often emerges, either in the form of a new way of approaching the existing opportunity for which you had designed plan A, or in the form of a new opportunity altogether.