I can't change shutter speed

This sounds like a hardware issue, it's likely that the shutter wheel is damaged. If you have recently purchased the item, I'd recommend requesting a refund. I would not recommend attempting to repair an issue like this yourself unless you are okay with destroying the camera or really know what you’re doing.

You could try a factory reset the camera software may be corrupt. To factory reset use the following step,

Factory Reset *Press “menu” button to the left of LCD screen. Select “shooting menu” or “Setup Menu”. Scroll down until you find “Reset Shooting Options.” Then press “OK”, highlight “yes” and press “OK” again. NOTE: This will reset all of your other personal settings. You will need to adjust other settings you have previously adjusted. *

It looks like you need to contact Nikon service. But you could try disabling Auto ISO:

Menu→Shooting Menu→ISO sensitivity settings→Auto ISO sensitivity control, and set that to OFF.