If you use more than one computer to collect items into your Zotero Library you will need to "sync" your library to Zotero's server. This will allow you to access all of your items.
First you will need to open your Zotero Library prferences
Provide your Zotero account username and password. Select "Sync Automatically" and both choices under File Syncing. This will ensure that all attachments are also updated.
For more information on syncing and troubleshooting assistance, go to the Zotero website.