Accidentally rotated text in the bounding box - How to restore to level?

You appear to have two text boxes overlapping.

There should not be two boxes around text. All text should be in a single text frame, whether it's the main flowing-text frame on each page, or a separate text box. A separate box should only be used to hold a small(ish) amount of text so that it can be styled as, say, a pull quote or callout, or a caption. In general, there should only be one text frame per page, with the main text flow in it.

Unless there is something more complex than a text page layout going on here, you need to delete one of those text frames; the one that isn't holding the text, the one not apparently rotated here, is a good candidate.

Once you've done that, you can either delete a rotated or otherwise messed up text frame on a page and drag a new one into place by clicking on the 'next frame' button of the prior page's frame — like this:

or, the easier method and the direct answer to your question, simply snap the rotated frame back to 90 degrees. Hover over the upper right corner and drag until the frame snaps to the margin guidelines. You can also fix it by selecting the rotated frame, and in the rotation field of the Control Panel at the top of the ID interface, set the value to 0:

Also, for all such "oops" situations... Ctrl-Z is your best friend. 🙂