Spreadsheets within a file reformatted without prompting from me

Hello Anne,

Thank you for posting question on Microsoft Windows Forum!

Based on the issue description. Well! One of the possible causes for this is Default Printer Change. If your default printer was changed (e.g., to a new network printer or a PDF driver) that has "Portrait" as its hardcoded default, Excel may re-evaluate the spreadsheets to match that driver’s capabilities. Also, a recent update to Microsoft 365 or a pushed GPO setting regarding printing/Office templates could have reset the default global Book.xltx or Sheet.xltx templates.

Another point worth mentioning here is the "Select All Sheets" Trap. If these files were edited by a user who right-clicked a tab and chose Select All Sheets, any change made to the Page Layout on one tab would have instantly applied to every sheet in that specific file.

To prevent this from recurring, check if your Default Printer was recently changed. If you use a "Print to PDF" driver as your default, ensure its internal settings are set to Landscape, or switch your default printer to one with more flexible driver settings.

Please note: Make a full backup of the folder (copy the entire folder to a different location or external drive).

If the files are stored/synced to OneDrive or SharePoint, you may be able to restore them to a previous point in time (version history or library/account-level restore). On the other hand, OneDrive/SharePoint offers version history per file, SharePoint/OneDrive for Business also has library/account-level “restore” features that roll many files back to a chosen date. This is ideal if the orientation change happened recently and you want the exact prior state.

Hope the above information is helpful! If it is. It is appreciated to consider clicking "Accept Answer".

Excel stores page orientation per worksheet, not per workbook or per installation. A bulk change from landscape to portrait across many sheets is usually caused by one of these:

1. A template or “master” workbook was changed
   • If new workbooks are created from a template that was edited so its sheets are portrait, every new file based on that template will inherit portrait orientation.
2. Grouped worksheets were reformatted
   • If worksheets were grouped (the window title shows [Group]) and Page Layout → Orientation → Portrait was chosen, that change applies to all grouped sheets at once. If that workbook was then used as a base for many others (Save As / copy), the portrait setting propagates.
3. A macro or add‑in changed Page Setup
   • VBA code or an add‑in can loop through worksheets and set PageSetup.Orientation = xlPortrait, affecting many sheets or files without individual prompts.
4. Printer or driver changes
   • Orientation settings are tied to the active printer. A printer/driver change or reset can cause Excel to recalculate page setup, which may revert sheets to portrait if the template or default is portrait.

Undoing the change

Within a single workbook:

• If the change just happened and the workbook is still open, use Undo repeatedly (Ctrl+Z) to revert orientation changes.
• To set multiple sheets back to landscape in one action:
  1. Select the first sheet tab.
  2. Shift‑click the last sheet tab to select all sheets that should be landscape (or use Select All Sheets from the sheet tab context menu).
  3. Go to Page Layout → Page Setup → Orientation → Landscape.
  4. Ungroup sheets (click another sheet tab or Ungroup Sheets).

This updates all selected sheets in that workbook at once, not one sheet at a time.

Across many workbooks (150+ files):

• There is no built‑in single command to revert orientation across multiple separate files.
• Each workbook must be opened and updated; however, within each workbook, grouping sheets lets orientation be fixed in one step as above.
• To avoid future repeats, create or fix a template that uses landscape by default and base new workbooks on it:
  1. Create a new workbook.
  2. Select the worksheet(s) to be landscape.
  3. On Page Layout → Orientation, choose Landscape.
  4. Save this workbook as a template and use it for future files.

If a macro or add‑in caused the change, disable or adjust it before correcting the files, otherwise the issue may recur.

References:

• Print an Excel worksheet in landscape or portrait orientation