Ever seen a sheet with what looks like missing gridlines? Wondered what happened? The issue is a format.
If you have seen something like the image below and wondered what is going on, the answer is a Fill colour.
It is caused by the White Fill colour, which clears the grey gridlines.
Some people use it because they don’t realise you can turn off gridlines – see the View ribbon tab
To get rid of it just select the range and use the No Fill option.