Errors were detected while saving 'filename.xlsx'. Microsoft Excel may be able to save the file by removing or repairing some features. To make the repairs in a new file, click Continue.

What's actually wrong

Excel failed while writing the file: either something in the workbook can't be serialized (a corrupt pivot cache, broken VBA project, damaged external connection, add-in interference) or the destination interrupted the write (flaky network share, OneDrive sync grabbing the file mid-save, disk-full). Your unsaved changes are what's at risk right now — the copy on disk is usually still the last good save. If this happens every save on one particular workbook, that workbook is carrying internal corruption that will eventually bite; think-cell and other add-ins are documented triggers too.

Free fixes — try these first

1. Rescue the session first: don't close Excel. File → Save As → choose your local Documents folder (not the network drive/OneDrive) → new filename. If that works, your changes are safe; deal with the original location afterwards.
2. Click Continue and let Excel save a repaired copy to a new file, then read what it removed and compare against the original before adopting it.
3. Rebuild by moving sheets: right-click a sheet tab → Move or Copy → To book: (new book) → check 'Create a copy' → select all sheets. Save the new workbook — this leaves the corrupt internal structures behind (named-range junk, dead styles).
4. Save as .xlsb (Excel Binary Workbook). The binary writer frequently succeeds where the XML writer errors, and it doubles as a compact backup of the current state.
5. Isolate the trigger: restart Excel in safe mode (Win+R → excel /safe), open the file, try saving. If it saves, an add-in (think-cell is a documented case) is the culprit — disable add-ins one by one via File → Options → Add-ins.

If the free fixes fail

If Excel crashed or was closed mid-save and the file on disk now won't open — half-written ZIP, missing parts — the free routes above can't help because the on-disk structure itself is torn. Our analyzer inspects the archive to determine which sheets and records survived the interrupted write and reports the recoverable percentage before you decide anything.