I mistakenly deleted some row and column lines, and would like to get the page back the way it was.
If you did not yet close your file you can Edit -> Undo (CTRL+Z) up to 100 of your last actions.
In the meantime, if you made other changes that you need to keep, first copy the new data, paste it in a new document, then proceed with the Undo.
undo: having 100 steps available depends on the setting for that, 100 is default,
to save the actual work use ‘save a copy’, it shouldn’t touch actual data and undo capabilities,
if closed: look for all possible backups (automatically … ‘always create *.bak’, external and so on and make separate copies of them as well as the file as of now, before! trying any other steps,
enhancement proposal: a possibility to save a file with undo info / steps integrated would be great against such mistakes, and be great in the risk of ‘unintended undisclosing’ data when giving a file away,