The parameters toolbox dissapeared. Again.

This happened before with an earlier build. The little thing in the tools that lets you control the properties of the tool you use (for example, number of subdivisions when you subdivide). Closing and reloading doesn’t bring it back, and it’s there when I open a new blend file.

Is there a key you press to bring it back, or something?

Look in the lower right corner of the 3D Viev there should be a small plus icon, click it.