This is probably bad news. If the character was missing, then it would have meant it was deleted and you could potentially recover those files. Now, it means that those files are corrupted, which is why the character doesn’t show up in the game.
You can use a hex editor and examine the .d2s file to see how badly corrupted it is. Here is a post I wrote about it: Offline - Character Disappeared and Cannot Start Any Games - #6 by TimberWolf-1533
Unfortunately, all the people that had this happen and had examined their file noted that the file had been completely zeroed out. That means there is no chance to recover it, as all information inside was wiped. I have a bad feeling that this is what you will find if you examine your file.
Why this happens is beyond me. I hope Blizzard is aware of this issue and is working on a fix. In the meantime, I suggest regular backups of your save game folder.