Welcome to Android Central! So you're connecting via USB cable, right? In your computer's file browser, can you actually see the folders and folder contents of the phone? If not, make sure the phone isn't screenlocked. Then swipe down the notification shade, tap the USB connection notification, and make sure it's set to MTP (or USB Disk Drive, if available).
If you've done all of that and you're still having problems, consider using a different cable, since a cable can fail at any time.