Don't panic, chances are your data is still there. This can generally happen if you logged into the wrong IMDb account.
When you add titles to your Watchlist, vote or review, this is associated with the IMDb account you're logged into. If, for any reason, you create and log into a different account, all this information won't be there. A second IMDb account can be created by accident when you may have an existing account, but then log into IMDb using a different login method (i.e. Amazon, Facebook, Google, or Apple). When you login using these other methods, it will create a new IMDb account with your login credentials.
To view this information, you'll need to log out of IMDb and log in again using the same email and password for the IMDb account where you created lists or rated titles.