We're sorry to hear you lost items after not logging in to Adopt Me for a while! While this is a rare circumstance, items can still go missing due to the following reasons:
Data Loss
Here at Adopt Me, we can only hold player data for a certain length of time to adhere to the laws of countries where Adopt Me is playable. This means that we cannot track data older than this, and you should log in to Adopt Me regularly to ensure your saved data is up to date.
Hacking
If you have taken a long break from Adopt Me or Roblox to find your items missing, your account may have been compromised while away. If you log in and notice trades you didn't make or any strange changes, this may have been the reason. It is important to make sure that your Roblox account has 2-step verification protection, and if you believe your account has been hacked, that Roblox has secured the account to ensure that no other parties can access your account even after a password change.