Files may go missing in alfresco. They can be found in the contentstore.deleted folder. If the file is still there, but the program you try to open it in indicates that the file is corrupt, try opening the file in a text editor. This will indicate the path and filename that the data file will be in within the alf_data/contentstore.deleted folder. It can be copied to a desktop and renamed to the original name. It will then be openable, and can be uploaded back to alfresco.
The file will look something like the following:
The node's content is missing: node: workspace://SpacesStore/7c7ace45-5ff5-49b4-a973-cfa9cd637d68 reader: ContentAccessor[ contentUrl=store://2009/8/3/19/37/4f5ef1b7-a524-4d1a-85e8-6e66d1b094f9.bin, mimetype=application/vnd.openxmlformats-officedocument.spreadsheetml.sheet, size=0, encoding=UTF-8, locale=en_US] Please contact your system administrator.
In this case, the file is 2009/8/3/19/37/4f5ef1b7-a524-4d1a-85e8-6e66d1b094f9.bin