Hi little gecko,
Got there one definite case of DWG format data corruption. We do data restoration service but it can be a bit expensive.
See if you can open the file you saved as AutoCAD 2004 2005 file format and work on from that?
As a tip, with any important drawing files I would run "audit"
and "purge" before
finally saving them - at least once a day - when you make changes in it. It doesn't hurt the file (at least when you select N to fix errors), and small incremental housekeeping can save you from a big problem (like this one) from developing later on.
IMO also keeping a running archive of at least three prior drawing versions (many keep far more) for each major file also is useful when you discover problems developing in a drawing so that you can at least go back to diagnose what went wrong. With the availability of cheap, reliable external USB drives on the market "not enough hard disk space" is not an excuse
As a point of interest, most major CAD system offer data housekeeping tools. Mainly because CAD data structures become very complex, with memory pointers and reference handles, coordinate math transformations and object containers etc so storage errors sometimes occur under the limitless combinations of features users create in their drawings. AutoCAD introduced "audit" and "purge" many versions ago. CATIA V4 had a "catclean" IUA script for this purpose. And so forth.