Text overflow occurs when a single word exceeds the width of the frame. This typically causes all of the text to disappear inside the text box and an alert to show above the canvas next to undo and redo.
To resolve this issue you can resize (hint: pull out on the white circles) the text box to add more width.
If that doesn't work you can hit ctrl + a (command + a on Mac) to select all the text in the text box and then proceed to lower the font size to recover your text.
Notice: Text overflow also occurs if your text box is at the bottom of the canvas and pushes the text outside of the canvas. To resolve this simply move the text box up or resize the frame until all the text is visible.