This is a known bug that happens from time to time. The drag and drop feature gets confused about where you wanted to drop a creature. It is supposed to warn you when you are overwriting a character with a compendium-dropped monster, but every once in a blue moon, it skips and instead just plays havoc with your sheet. If you are using the D&D 5e by Roll20 sheet, you can likely restore most of your character simply by reverting to PC. Many of the attributes are stored differently. I would still recommend using this as the basis to re-build the sheet from scratch. I wish I could tell you how to prevent this happening in the future, but it seems to be random. Fortunately, it is also rare. You can try contacting <a href="mailto:team@roll20.net" rel="nofollow">team@roll20.net</a> and request a rollback, since this was a glitch and not an error on your part. It may undo other work you've done since, so it's up to you.