According to the manual for your camera, if none of the indicators are on when the shutter release is half pressed, the batteries are exhausted. If the 3 lights are on at the same time, exposure is correct, if only the plus or minus is displayed under/overexposure is present.
Be sure to check your film speed settings, as even if your camera is telling you your exposure is correct, if the film speed isn't correctly set, your images will not be properly exposed if the camera is not set for the film actually loaded in it. Otherwise, check your batteries.
Also, keep in mind some lighting conditions can trick your meter and may need to be compensated for. If your camera has an exposure compensation dial, which yours should, check to be sure it is set to "0" as this could also cause your exposure issues if it's been moved previously and never reset.
Here's a link to your camera's manual, unfortunately it's not very visual so it's hard to get a very good idea of exactly what features your camera may or may not have.
http://www.cameramanuals.org/pdf_files/vivitar_v3800n.pdf
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