Reset the audio-connection cable you are using to record audio to check for a loose connection. The Portable USB Recorder has RCA (red and white) audio inputs, so whether you are recording from an RCA source device or using an RCA patch cable to record from a non-RCA audio device, unplug the cable and plug it back in and again attempt to record the audio.
Replace the audio-connection cable you are using to attach the source device to the USB recorder with another one. A faulty RCA connection cable is by far one of the leading causes of a failure to record audio or intermittent audio interference during recording and playback. Attach the new connection cable from the source device you are recording and re-record to test for any improvement.
Use an alternate USB mass-storage device if you are not able to play back the audio you recorded. Having mostly ruled out problems with an audio-connection cable as causing the problem, a faulty USB storage device is another common cause of difficulty saving or playing back the audio signals you recorded. If no data is being saved on the other USB device, you may need to try recording from a different source device or have the USB recorder serviced.