On your device, when connected to your PC, slide down your notifications and make sure "USB Mass Storage" is what's enabled.

 

When i bought my phone(Samsung galaxy pop) , when I plug in my Droid to my computer I used to get a full screen pop up with little green Android dude and message

"USB connected" with button for "Turn on USB storage" at the bottom.

but now this is not happening...I dont get any notification as such except the tethring notification that the usb is connected. I'm unable to transfer files from PC or to PC.
Please help.
Thanks!!

 

OK, after facing similar problem, I have called the store I bought my Samsung Ace 2, and the sale lady gave me this solution and it worked perfectly:

1) Unplug everything.
2) Go to your phone 'setting' -> 'wireless and network' -> 'usb utilities' then press the button 'connect storage to PC' THEN plug in the cable to the phone & computer. Then you will see android logo and it will say USB connected. Then your computer will detect the driver. You will have to repeat these steps every time you mount your usb driver.

Hope this helps. I haven't seen any solution on any forum sofar. If it works for you, please report back too.