coz your d80 unable to focus at the subject.. shutter will not close unless the AF light at the viewfinder is lighted up... try changing to MF or shooting something that is the same distance as your subject..
The camera did not achieved auto-focus and will not release the shutter. Simple way to overwrite this function is to switch to manual focusing. Shutter will release regardless of focus.
Do take time to read through the manual if you are new to the dslr