Welcome to Android Central! First, swipe down the notification panel, tap the USB icon, and make sure MTP is selected, not Charge Only.
If it is set to MTP, or if nothing shows up at all, make sure you're using the original cable. If you are, then perhaps the cable has failed--try a different cable.
If multiple cables don't work, try a different USB port on the computer, and also check the phone's microUSB port for any debris or damage.
Using a cable that comes with a power-only device, like a wireless headset, will also do that - they only have wires on pins 1 and 4. (Headsets don't do data over USB, so they save a few tons of copper a year by leaving those 2 wires out.)