Many of the files on your Chromebook device are still available when the device is not connected to the internet.
While procedures differ for each application, Google Docs, for example, can be accessed offline in the same manner as if it were online, including the creation of new documents. Offline copies of each "native" Docs file (other files, such as PDF documents may not be available) can be opened, edited, and saved. The next time the device reconnects online, the changes will sync to Google's cloud storage.
Tip: In cases where internet connectivity is poor or intermittently unavailable, Google Docs may not realize that it is "offline." It may be necessary in those cases to disable the wireless connection in order to access offline content.