GNU Emacs Manual
AD.9.4 Garbage in the Text
If C-l shows that the text is wrong, try undoing the changes to it
using C-x u until it gets back to a state you consider correct. Also
try C-h l to find out what command you typed to produce the observed
If a large portion of text appears to be missing at the beginning or
end of the buffer, check for the word `Narrow' in the mode line.
If it appears, the text you don't see is probably still present, but
temporarily off-limits. To make it accessible again, type C-x n
w. See section AC.22 Narrowing.