Python:“ModuleNotFoundError"，但安装了模块?

A minimal (non-working) example:

import matplotlib.pyplot as plt

plt.plot(range(10))
plt.show()

返回以下错误:

C:Anaconda3python.exe C:/Users/John/Documents/CPU/master/untitled/main11.py
Traceback (most recent call last):
  File "C:/Users/John/Documents/CPU/master/untitled/main11.py", line 1, in <module>
    import matplotlib.pyplot as plt
  File "C:Anaconda3libsite-packagesmatplotlibpyplot.py", line 29, in <module>
    import matplotlib.colorbar
  File "C:Anaconda3libsite-packagesmatplotlibcolorbar.py", line 34, in <module>
    import matplotlib.collections as collections
  File "C:Anaconda3libsite-packagesmatplotlibcollections.py", line 37, in <module>
    import matplotlib.lines as mlines
  File "C:Anaconda3libsite-packagesmatplotliblines.py", line 28, in <module>
    from matplotlib.markers import MarkerStyle
ModuleNotFoundError: No module named 'matplotlib.markers'

Process finished with exit code 1

两周前运行良好，但现在不行.据我所知，我没有改变或更新任何东西.模块加载正确，但似乎是模块内容发生了变化?如果是这样:这是怎么发生的，我该如何解决?

(C:Anaconda3) C:UsersJohn>conda install matplotlib
[...]
ERROR conda.core.link:_execute_actions(337): An error occurred while installing package 'defaults::matplotlib-2.0.2-np112py36_0'.
OSError(22, 'Invalid argument') Attempting to roll back. 

OSError(22, 'Invalid argument')