frame not responding

[visha123-sys]

error Traceback (most recent call last)
in
20 flag=0
21 while True:
---> 22 ret, frame=cap.read()
23 frame = imutils.resize(frame, width=450)
24 gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

error: Unknown C++ exception from OpenCV code