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.