Hello jaganXDA85! This has happened before to me and after several futile attempts, I've found out that the USB cable I was using was not the proper one, so try the one that came with your phone since it's the properly designed specifically for it or if you don't have it anymore, then try out another one that's originally made by the same manufacturer. If you can't find such, then you'll need to keep trying with different cables until you find one that's working.
If you have the original one and still doesn't work, then either the cable itself is broken (you'll be surprised but it does happen) or the connection to that particular port is faulty, either there's some oxidization happening within your cable's terminal or the one port in your computer; you can try it out in different ports if you have more than one.
---------- Post added at 11:07 PM ---------- Previous post was at 11:02 PM ----------
Sorry to hear that widget2k13! But I can tell you that my S8 is working just fine so I'm pretty sure that this is the way to do it. Maybe there was a combination of everything I've tried; I remember that I did the ADB command lines, reboot it several times and didn't work. Then I did install the software I suggested before and after rebooting another 2 times it worked! Keep at it and eventually it'll happen, don't give up! This is a really wonderful phone and since I've started using the original Samsung case, I've solved the one fatal flaw I've found which is... wait for it, yep the fingerprint reader in the back! Now thanks to the special design on the case, there's a little bit of tracking between the camera opening and the reader, enough that I don't have to keep searching for it and if I can just wait half a second more it works almost 9 out of 10 times.