When copy-pasting from a non-Pages document you sometimes get ‘crushed lines’, when there is no interval between the lines and they overlap one another. Like in the picture.
To return text back to normal, select the whole ‘squashed’ passage, go to Text Inspector and in Spacing>Line click on the drop-down menu under the line spacing value window.
Change the setting from Exactly to Single. This changes the setting from points to lines and a measurement of one corresponds to the one-line interval between the lines. In the source document it was probably set in points, not lines.
The text will now have the usual one line interval between the lines.
And of course – don't panic! Nothing's wrong with the computer or the programme.