Denaii

A friend of mine created a server with the Tekkit Lite mod pack about a week ago which a few of us have been using. We've had no problems the past few days until today. The server was opened this morning and everything was fine, the server was closed for about an hour as my friend had things he had to do but when he reopened the server half our world had reset and all of the blocks from the mods had gone yet the standard blocks (e.g. torches, cobblestone, netherbrick, etc... ) are still there. We had built a tower in the middle of a mountain where we had macerators and other blocks from the mod pack with a huge smelting sytem as well as a nuclear reactor and the mountain has reset back to it's original state even the wooden planks and stairs have gone. He didnt update the modpack or mincraft and he didn't try adding new mods so we don't understand what could of caused this. Does anybody know why this had happened and how to prevent it in future? Thanks in advance :)