Assuming that you're seeing "ERR" in the viewfinder display, there are a couple of common problems associated with this message. Check the LCD screen for additional information, such as "Start-up error" or "Auto-exposure error." Often the problem can be resolved by removing all accessories, the memory card, and the battery, then re-inserting them. See the [[Nikon D5000 Troubleshooting|Nikon D5000 troubleshooting page]] for more details.