Could you please try:
– Turning off all your plugins. Is the issue gone? If it is, turn them back on one-by-one to find the conflict.
– Switching to a default theme like Twenty Fourteen or Twenty Thirteen and see if your menus work as expected. You may need to reset your widget and other theme-specific settings afterwards, so you might want to make a note of them.
– Using a different browser. If it happens only in one web browser (like Firefox or Chrome) then try deactivating all of that browsers add-ons or extensions. If the problem goes away, then you know an add-on is to blame and you can try turning them back on one by one to determine which one is at fault.