HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

There aren't many options here:
1. Reinstall Heroes (download this mod from the official website, preferably the latest version).
2. Try downloading other Heroes of Might and Magic maps (maybe the problem is with the map? This is not uncommon for Heroes of Might and Magic).
3. If everything was fine before installing the HD mod, and errors appear with HD, then you need to choose between HD or LoRD.