This is most likely normal autofocus behavior in low light, not a camera fault. After the reset, a setting such as the AF-assist illuminator may have changed. If AF-assist is off, the D3100 may struggle to focus in dim conditions and may refuse to release the shutter until focus is achieved.

Things to check:

• Turn AF-assist illuminator on.
• Confirm whether your current focus mode/release priority requires focus lock before shutter release.
• Try a mode like Sports, which may allow shutter release without perfect focus.

Why exposure can look OK but it still won’t shoot:

• Exposure and autofocus are separate. The meter may indicate a usable exposure, but the autofocus system can still fail if there isn’t enough light or contrast.

Workarounds:

• Use the built-in flash or another light source to help focus.
• Briefly light the subject with a flashlight/phone light, focus, then take the shot.
• Use Live View manual focus if needed.
• Focus, then switch the lens to manual focus to lock that distance before shooting.

So: check AF-assist first, then focus/release behavior settings.