1. If you saved at least once then Pages should have auto saved regularly and you can save most but not all of your work when you reopen the file.
2. If you never saved, take a screen snap of what is visible: command shift 4 and drag over the window. That will be saved to your desktop.
3. To have a better chance of recovering your work next time, always save when starting a document and set up Time Machine on an external drive.
4. Look at why there may have been a problem. Too little space on your hard drive is one, too many Apps open is another, particularly a browser with lots of tabs open and too long a document is also a risk of losing your work. Especially do not save to iCloud, USB flash drive, or an external server. Always save to a good hard drive with plenty of room left.
5. If you have important work DO NOT USE PAGES! Particularly as I suspect you are using either Pages 5 or 6.
6. Use something more reliable that lets you parse your file if you get in trouble, like MsWord or LibreOffice [free]
Lastly when you have a problem and seek help here, it really helps to tell us what version of Pages and what version of OSX you are using.
Peter