This points to a hardware fault, most likely the shutter mechanism or possibly an aperture/lens communication issue. The normal live view suggests the sensor is working, but if the recorded frame is black while the shutter sound has changed and you’re getting a restart error, the camera likely isn’t completing the exposure correctly.

Because it began after beach use, sand, moisture, or corrosion could also be involved. Don’t use compressed air inside the body; it can push grit deeper or damage parts. Try these safe checks:

• Test with a fully charged battery.
• Remove and remount the lens; clean the lens/body contacts gently with a dry microfiber cloth.
• If possible, try another compatible lens to rule out a stuck aperture in the kit lens.
• Reset camera settings if the menu is accessible.

If it still gives black images or the same error, it likely needs professional service. Given the intermittent buttons and abnormal shutter behavior, this is not likely to be a simple setting issue.