回答“What is wrong with my camera's flash?”The shutter sensor pin may be stuck. The fix for this can be found on the Nikon D5000 troubleshooting page, among other possible solutions. If you've used the flash several times consecutively, the camera may be automatically disabling the flash temporarily to protect it.
回答“What does the "Err" error message mean?”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 page for more details.
回答“Why is the battery icon blinking?”Are there any messages being displayed on the LCD screen? Usually the camera will give you some more information on the problem ("Battery cannot be used," "Initialization error," "Shutter release disabled") These three situations are described on the D5000 troubleshooting guide.