I cannot find the solution to this problem in any of the tutorials.

After I created an object, I created a Bezier Curve to create a second object and turned it into a mesh. Now, I cannot edit the original object. How can I make the original object editable again?

Tab in to Object Mode, select you object then tab into Edit Mode.

I have tried switching from object to edit mode but I still cannot edit the original object even in edit mode.

Is there something that prevents a person from using two different object types in one file? Example: I created a cube using Space, Add, Mesh Cube. Then an Bezier Curve. Even when I turn the Bezier Curve object into a mesh I seem to have a problem editing the former cube.

Quote:

Is there something that prevents a person from using two different object types in one file?

That would be a rather strict limitation, don't you think?

Conceptually, this is simple.
1) select a single object
2) tab into edit mode

Maybe if you show us the .blend

did you by chance disable Select in the outliner? (The cursor icon in your scene tree)

Problem solved. I was selecting by using the B key which caused the problem. Directly selecting with the right mouse button solved it.