I don't have a usb modem but if I have problems 1st thing is to reset the modem (which for mine is either accessing it via the browser or else disconnecting the power for a few mins.) At worst I'll disconnect the modem & reboot the computer. Modems can apparently stop talking to the computer. But then I often run so many apps at he same time doing heavy duty processing it's no wonder. I process graphics & video mainly.