I've suddenly lost the ability to type in newly created text boxes.

Hi @dazapper:

InDesign doesn't prefer "500 pt text"—some sort of default has changed in the file.

My guess, without seeing your document, is that you are not taking full advantage of paragraphs styles yet. This is something to look into when you have some downtime.

I'm also guessing that your text is using [Basic Paragraph] in the Paragraph styles panel or pehaps a paragraph style based on that definition and that definition is what is changing. You can right click it > Edit to modify the definition. 

My guesses could be completely off, of course. Screenshots help us help you more quickly, as does sharing your file.

~Barb

Thanks for the guidance, Olivier! For some reason unknown to me, the text default size reverted to "0," effectively making any type useless. It's working fine now, thank you for the advice. It was only happening in this particular file, it was fine in another book I'm working on.  

@dazapper This can happen if the textbox is too small to display any text in the default font size (this issue is particularly egregious in tables). Try to make your textbox larger, change the font size, and then restore its original size. Alternatively you could change the font size of your default text style.

Is this in one file or all files?

~Barb