Setting up account syncing between your desktop and web versions is really important. If you don’t do this, and don’t use it, then you may lose references you have added when you log off.
To set this up on the desktop version:
It is good practice to regularly sync when using Zotero, and you can do so by clicking on the round green arrow in the top right of the program window. If you are using a College PC, make sure to enter your Zotero username and password in the Sync tab every time you sign on to the computer, or your references will not save.