I've just run into this myself. I don't think you can recover it, currently. I haven't succeeded. (Update: reported bug and found a workaround: see below)

LastPass has useful features to let you check deleted entries, and each entry has a "History" for both username and password.

But if you replace the entry for a site, you miss both of those safeguards. The old entry isn't saved in deleted sites, it just vanishes and is completely replaced (so no history of the old username/password is saved anywhere, either).

UPDATE: we have a LastPass enterprise subscription, so I got tech support on this. They now know about the bug (so hopefully it will be fixed), but they also had a solution -- you can revert your entire vault to a backup from ~25 hours earlier. Normally this is to recover when you change your master password and can't remember the new password, but you can do the same thing to recover from LastPass bugs that cause data loss.

https://lastpass.com/revert.php

The text is all about reverting a master password change; proceed and you'll get a notice that says "We do not have a record of any recent LastPass master password changes for your account. Alternatively, you can restore your account's state as it existed ~25 hours ago."

That's the option you want.