Shutter does not function

Make sure there isn't a dial lock feature engaged. In some models, you can lock out the dials to prevent accidental changes while shooting.

Difficult to go on only does not make sharp pictures anymore.
Seeing one picture says more than 1000 words. Still I give it a try.
When you are sure you did not change the focus settings and don't switch over to macro settings, while shooting normal, I can't help. But I do advice to look in the manual on page 41 and put the camera in Auto focus. (Not one of the macro modes) then when shooting, wait till the camera has focused, while you pressed the shutter realise button half. When the camera is in focus, the square in the middle of the screen becomes green, press the button down. (without moving the camera)
Also select a high resolution (big photo size) because small photo size is easy seen as a blurred picture.

Success

I could be that your lens isn't focusing. If the Auto focus can't find something to focus on the shutter won't fire. If you are shooting in low light sometimes it has trouble try shooting something that is well lit. also your auto focus point may be somewhere other than the center. there is a button on the top of your camera that looks like this [ ][ ] [ ] [ ]
More or less that will switch where your focus point is. If you look in the view finder and see a red dot when you push on the shutter button that is your focus point. push the above button until the red dot is in the centre. that may fix your problem.

I'm going to assume at this point that the camera has been serviced correctly. What I'd suggest is that you look in your manual and find where you can reset your camera back to the factory default settings. Okay here are a few thoughts that might be what is happening again I'm assuming the service was done correctly. In "Auto" mode the camera is in full control and what might be happening is the camera has selected a shutter speed many seconds long to achieve a correct exposure, most DSLR's will have a 30 second shutter speed. Also the mirror up function my be activated so first the mirror comes up then the shutter delay happens. The mirror lock up function on my fleet of digital cameras locks up every time I trip the shutter until I set the function to off. It'll really throw you off if the mirror lock up is activated, the camera is set for a time delay be this 5 or 10 seconds and then you have a long exposure on top of that, feels like forever before the picture is actually taken. By going into your setting or custom functions and setting the the camera back to factory default so bring you back to the setting the camera had when you first got it.

are you shooting in raw format? and in hi resolution or low resolution ? If you are shooting in RAW then the buffer of the d100 is full and it needs to empty its buffer before you can shoot anymore pictures

Take it to a cell phone place and let them try a new battery.. My wife was talking on hers in a light drizzle, it shorted everything out one tiny tiny drop. The red light came on, never to work again, once the screen shorts out for a split second it is dead.If the battery shows red x's on the bottom or behind the battery you find a red x you can be sure the screen is shorted out.

check on the shutter speed and also the Aperture priority..

Hello parr_alp,
Please keep in mind that when shooting in low light, you will ALWAYS get some amount of blur when you are hand holding the camera unless you use a flash. The only way to increase your shutter speed is to crank up your ISO setting to 800, 1200, 1600, etc. If there is no tripod handy, brace yourself against a stationary object and remember to keep your elbows tucked down near your body when shooting (no sense in holding up your arms, too).
There is no way to increase the ISO limits of your camera while maintaining quality of your images. And once again, you will ALWAYS get a certain amount of blur when hand holding the camera. 
The external flash not firing is an entirely separate issue. This could have to do with your metering, a bad flash, bad connection in the hot shoe, what shooting mode you're in — the possibilities are numerous. The next time your flash doesn't work, write down all the settings of your camera and keep track of them (i.e. shooting mode, metering mode, flash settings, current lighting situations, etc.) You may start to notice a pattern and perhaps we can help you here.
Anyway, happy shooting, and if this response was helpful, please rate it! Thank you!
Jeff

Hello Ruchai - Do you mean specifically that it will not focus on these object specifically when in DIM light, or do you mean that it will not focus easily on red objects at all? This is important, because many of the flowers in people's gardens, including my wife's garden, are red. I shoot these all the time, and I have no trouble focusing at all. Now, if you are talking about dim light, it may be that the problem is the dimness of the light more than the color of the object. Pondria has done some interesting work studying that the camera's focus system may be 'seeing' everything as a b&w or grayscale tone in order for its autofocus system to work properly. I do not know if this the truth or not. But it the red color truly is a problem, perhaps there may be a workaround. Focus the camera upon a nearby portion of the plant -- the leaves, a stem, another flower of a different color, etc. Then half-press the shutter button. Finally, reframe the shot and fully depress the shutter button. I will try this today in my garden to see my results with red flowers and autofocus. I will attempt both the regular Autofocus as well as our lauded Spot-focus technique. :) Thanks for more interesting comments.

This is normal for the 750z.. a friend of mine has the same problem. If you try to print straight from the camera, everything is too bright. My friend has to manually lower the brighness and raise the contrast on every picture.