try to restart the adb following below commands:
"ABSOLUTE_PATH_OF_ANDROID_SDK\platform-tools\adb.exe" kill-server "ABSOLUTE_PATH_OF_ANDROID_SDK\platform-tools\adb.exe" start-server
I am also facing same problem and restarting adb fixed my problem. I have created a ".bat" batch file with above described commands, I run it every time when I am facing this problem.