Sony Xperia S - suddenly can't connect to PC anymore (it was working before)

I see there are various threads for Galaxy S3 having this problem, but my phone is Sony Xperia S. My phone used to connect to PCs fine, but today when I tried connecting to my laptop it wouldn't recognize it. This happens on Windows and on Ubuntu, with the original USB cable, and with a generic one. The same cables and computers used to work fine with my phone. Also, the phone does not give any notification that a PC has been connected, i.e. the phone itself doesn't recognize that a PC has been connected. It starts charging, though. I tried rebooting Android, I also tried enabling USB debugging, but none of it helped.