There are a few possible explanations for where the auto rotate feature has gone on your device. One possibility is that it has been turned off in the settings. To check if this is the case, go to the settings menu and look for a setting that says "auto rotate." If this setting is turned off, you can turn it back on and the auto rotate feature should start working again.
Another possibility is that there is a hardware issue with the device. If the auto rotate feature is not working and the setting is turned on, try restarting the device. If that does not work, you may need to take the device to a repair shop to have it checked out.