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If C-l shows that the text is wrong, first type C-h l to see what commands you typed to produce the observed results. Then try undoing the changes step by step using C-x u, until it gets back to a state you consider correct.
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 Narrowing.