First of all, you should use RAR recovery record when storing archives onto unreliable media such as floppies. Generally, it is not recommended to create solid archives in this case. You should use the normal (non solid) mode instead.

If you have a damaged volume of a solid archive, try to repair the damaged volume. It helps if you used RAR recovery when you created the archive. You may check for the presence of a recovery record in the archive by using the Show information command. Rename recovered volume, fixed.arcname.rar (if recovery record was not used, WinRAR builds only rebuilt.arcname.rar) to the actual volume name and try to unpack your archive again, starting with the first volume.