I worked on assigment 6 yesterday and it was still working. But now I started up the application and I only get this for a brief second and then nothing happens. 
I've tried restarting my laptop, using admin to start it up, reinstalling arduino but nothing works. It just does not open. How do I fix this? The last thing I did when I closed the programme was moving the map of the mpu650 display thingy to my arduino map, what probably was the mistake, but I don't know what I can do to fix it again. Thanks!
This seems to be an update-related problem that affects a lot of people (using the Arduino software).
What helps is to remove the files package_index.json and package_index.json.sig from the folder C:\Users\XXXXXXXX\AppData\Local\arduino15 (replace XXXXXXXX with your username).
To avoid the problem to come back (it might), temporarily disable updates via File > Preferences and disable the option to check for updates when starting.
If you have difficulty accessing the folder, you can also remove the files from the command prompt:
del AppData\Local\arduino15\library_index.json*
On a MAC, the command can be:
rm /home/XXXXXX/.arduino15/package_index.json*(replace XXXXXX with your username)
