Some time internal flash does not work in my nikon D3100 camera

'make model (what it is)' is useful in questions like this!

How to fix the popup flash on your camera (Canon DSLR)

Tutorial Geek https://www.tutorialgeek.net > 2013/02 > how-to-fix-pop... Put the camera into Automatic mode and pry your fingernail under the flash (see the photo above). Right now the flash is locked, so you will not be able to ...

Why doesn't my D3100 take photos when the built in flash ...

Photography Stack Exchange https://photo.stackexchange.com > questions > why-doe... Feb 18, 2023 - I have a Nikon D3100. It doesn't take pictures while the built in flash is popped up, but is working fine when the flash is down. What could be ...

1 answer · 2 votes: I'm guessing that the exposure settings are significantly different (darker) when the flash is up because it is expecting the flash to add a lot of light. ...
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What do you mean by flash mode?
Have you read the manual how to use the camera?
If nothing from there helps, try connecting it via USB and check if there is firmware update. If even that doesn't help, try acessing menu and doing hard reset from there.

If you have set the main dial to the green camera logo, with automatic in bad light, and the flash still does not pop up, you could start worrying. The mail mode dial is very easy to turn, even without knowing. It happens to me more often than I would like. My next camera must have a special button to lock this dial.
When you chaco the manual, you will see there are settings the flash won't pop up and even when it is up it won't flash, because that menu excludes the flash.

It's safe according to the list at http://www.botzilla.com/photo/strobeVolts.html

The fastest shutter speed with flash is 1/200 of a second. Due to the design of the shutter, the entire frame is not exposed at faster speeds. See the box "Shutter Speeds Available with the Built-in Flash" in the "Flash Mode" section of the manual (page 70 in my copy).

may be due to not using for long period

If camera shows no sign of abuse, play st-pid and take it back to where you bought it and tell them unit is defective and you want to exchange it for a working model.

There is no problem with the trigger voltage, which I assume is your primary concern. You may not get all of the features you would with a Nikon flash, but you should be able to use it with the D3100 just as you use it with the N80.

Night mode? As a professional I can set a camera to shoot at night by the settings thats why we dont use point and shoot cameras
go buy a Nikon D90, or D3100 and set the modes for night shooting
good luck.

Basically, you put batteries into the flash, put it into the flash mount on the top of the camera, turn everything on, select a flash mode, and start shooting.
The flash comes with a detailed manual describing its operation. There is a section in the camera manual describing the various flash modes. The SB-700 is fully compatible with the Nikon iCLS system, which includes the D3100.

Since you asked a general question, I can only give you a general answer. If you have any specific questions, please feel free to ask them.