Is the problem specific to a single physical port, or do you have the same issue in all USB2.0 and/or USB3.0 ports on the same PC?

What USB device or devices are you trying to connect?
Have you tried other USB 2.0 or 3.0 cables?

What I have encountered is that a frequently-used USB port may get worn down to a point where the contacts no longer make a good connection. Usually, unplugging and reinserting the USB device solves it.