Saved as CSV and lost everything — other sheets gone, formulas turned to values, leading zeros stripped, long numbers showing as 1.23457E+12

What's actually wrong

This isn't file corruption — it's the CSV format doing exactly what it does: a CSV is plain text holding only the values of the active sheet, so formulas, formatting, charts, and every other tab were never written to it. Separately, Excel mangles ID-like data on open: it silently converts '01234' to 1234 and 16-digit numbers to scientific notation (with digits past 15 replaced by zeros), and if you then save, the mangled version becomes permanent. Whether you can recover depends on whether an .xlsx copy or the original source still exists — the CSV itself cannot give back what was never stored in it.

Free fixes — try these first

1. Look for the original .xlsx first. 'Save As CSV' creates a new file — the .xlsx you started from is often still sitting in the same folder or in File → Open → Recent. Also check File → Info → Version History (OneDrive) and right-click → Restore previous versions.
2. Recover an unsaved copy: File → Open → Recent → Recover Unsaved Workbooks (or check C:\Users\you\AppData\Roaming\Microsoft\Excel\) — AutoRecover snapshots frequently predate the CSV save.
3. Reopen the CSV the safe way so you don't mangle it further: don't double-click it. In a blank workbook use Data → From Text/CSV, and in the preview set ID/ZIP/phone columns to Text — this preserves any leading zeros and long numbers still intact in the file.
4. Check the ground truth in Notepad: open the .csv in a text editor. What you see there is what the file really contains — if the zeros/digits are present, only Excel's display was wrong and the Text-import above recovers them; if they're already gone, re-export from the source system (bank, CRM, store platform), because no tool can reconstruct discarded digits.
5. Rebuild the light stuff last: your values survived on the active sheet, so re-add only the formulas and formatting you actually need rather than assuming total loss.

If the free fixes fail

A related trap: many 'CSV' and 'Excel' exports from web apps are actually mislabeled files (HTML or CSV wearing an .xlsx name, or vice versa) that throw format errors when opened. Our analyzer detects the file's true format and repairs mislabeled files on the spot, and when data genuinely never made it into the file, it tells you that honestly instead of pretending a repair is possible.