Hi @shahalamhelal ,
As you can take pictures OK even though you can’t “see” what you’re taking and they are alright when downloaded to a PC you most probably have a loose connection to the LCD screen or a faulty LCD screen.
With the camera turned on try shining a torch at an angle close to the LCD screen and check if you can detect an image at all.
If you can detect an image it means that there is a backlight problem with the LCD screen, either a loose backlight cable connection on the motherboard (see image in link to supplier below - small cable is the backlight cable), a faulty LCD screen or a faulty motherboard.
If you cannot detect an image then it means there is a faulty LCD screen or a faulty motherboard.
In either case if it is a possible faulty LCD screen, try changing it first as this is cheaper than replacing the motherboard
Here’s a link to the ifixit Nikon D7100 LCD Screen Replacement guide which may be of some help.
Here’s a link to a supplier -example only of a replacement screen. It is only shown to give you an idea of what it looks like (note the separate small backlight power flex cable coming from the screen) and what the cost of the part is. There may be other suppliers online that suit you better. Just search for Nikon D7100 LCD replacement to get results.