Some systems have front and back ports.
Make sure that the default Mic is set to the proper port if you have more than one.
Also, there are usually MUTE options for them as well, so make sure that it is not ticked.
Another possiblilty is the Mic itself, as some have volume controls on them or their wire.
Make sure the Mic's volume is not set at a minimum.
Yet another possiblilty is that the Mic is defective. (even if it is brand new)
Try it on another computer or try a friend's Mic on your computer.
That will verify whether it is the Mic or the computer that is the problem.