Something probably went wrong encrypting the file. Do you know if you're using 2.50.0 or 2.50.1? If you don't know, check Help > About in ZC or Etc. > About in ZQuest. Build 24 is 2.50.0, build 28 is 2.50.1.
If you have any .tmp files in your ZC directory, one of them is probably the save file. If you send those to me, I may be able to recover it.