Use Ctrl-A, then right click on any column heading and select "Unhide". If
that doesn't work, the columns may not be hidden, and may actually just be
very slim, so try Ctrl-A and set the width of all columns using rt-click and
"Column Width", choosing a number like 10 or 20... Also, you may have the
Window set to have frozen panes, so try Windows | UnFreeze Panes as well.
They actually may not be hidden but they may be sized
down. Say column D is not visible, select C through E and
then go to Format>Column>Width and enter 10, you can then
go back and resize to what you need to.