I'm having a similar (but not as bad) problem, but I think I have the cure.
If you have Blockland Glass or an auto-updater, try turning it off/on.
If you don't have Blockland Glass, try moving add-ons in and out of your add-ons folder and turning on the game to see if anything fixes.
If none of these work, try taking out the files that are gone, opening blockland and out, then put it back in.
If you don't have the files, that's the problem you are having. Re-install blockland (If you need/want to) and check if the default add-ons are there. If not, there may be a bug with blockland's download or you may have a virus/bug on your computer.
If none of these work, message me and I'll try to help more.