Could you try the following steps:
1. Reboot the device by removing the battery.
2. And if that does not help, reset the device to factory settings; select Menu > Tools > Settings > General > Factory settings. To do this, you need the lock code. The default lock code is 12345. After resetting, the device may take a longer time to power on. Documents and files are unaffected.
This could be a hardware malfunction if the error still appears after the reset device, when starting the camera. In that case, the Nokia E71 device should be repaired in your local Nokia Care center.