TECH: weirdness???

One of my files, which I've been working on in Wordpad, has suddenly
acquired a vast amount of strange symbols when I open it---

row upon row of blank boxes
row upon row of ÿ (y-umlaut) with the occasional thorn interspersed.
odd series of letters, odd lists of characters (as if I'd been playing with
the Alt-Num stuff)

Fortunately the text is still there, in the middle of all this stuff.  It is
also duplicated later on, after another series of blank boxes etc.,  but
with things like ç and ñ replaced by odd characters.  What on earth is going
on?  It is the only file so affected.

(It is possible I originally wrote the file in WP8, then transferred it to
Wordpad-- but I don't think so.)