Nikon D3100 focus driving me nuts. How do I get it focused?

I have been trying to get the camera focused all day long and it's driving me nuts. It was absolutley perfect yesterday and now today it's off focus!

I'm shooting a largish stationary subject indoors, the lighting is good. I stand about 6ft back with the use of a tripod and the subject is about 4 ft forward from the wall.

I have tried everything I can think of… Also read online but with no success… So has anyone got any suggestions/tips/settings?

these are my settings:
Aperture A
ISO 100
AFS-single servo
AF Area-single point
Metering-center weighted
shutter speed 1
Aperture 5

Thanks

Added (1). The subject is a mannequin by the way…

Maybe it is the subject you are trying to focus on. Is it a flat subject? Try placing something like a soda can, a cereal box, anything colorful in front of your subject. Then try to focus and see if it works. Most of the time flat subjects with no contrast or lines has difficulty focusing.

Make sure that the cameras focus point is on the spot where you want your focus.

To get enough depth of field on a three dimensional object (like a mannequin), you need to stop the lens down at least two stops, maybe more.

Since you are probably shooting indoors, so using a sturdy tripod will assure that there's NO camera movement which can cause blur

What do you mean by this?

shutter speed 1 > Do you mean a shutter speed of one second? If so, camera movement is causing the blur

Aperture 5 > Do you mean f/5.0 or maybe f/5.6?