If you've got the original film see if the film is actually blown out.
If not if you have control of the exposure/gain on the scanner try a manual exposure.
I forget off the top of my head but for negative film I believe you want to increase the exposure to preserve highlights. The highlights are a dark area on the film, you need to increase the exposure to see into the "shadows" on the negative.
Once you have all the data you need it should be easier to fix the final picture. If you just can't get everything then you would maybe want to do multiple scans and piece it together.