My kobo touch has just upgraded to firmware 2.3.1, and I started getting this problem too. (There may or may not be a connection).
It was happening to me on epub files, though, not plain-text.
I turned the kobo off (hold the top switch until you get the black bar on screen that says SWITCHED OFF), left it for an hour or so while I had dinner, and when I turned it back on, it suddenly worked!
Let me know if this works for you; if this happens again, the kobo's useless to me and I'll have to leave it behind when I go away on vacation tomorrow