If you changed your Windows username, then you are probably receiving an Error 9, unauthorized user.
By far the simplest way to deal with this is to change your username back to what it was before.
Warning: if you deactivate and reactivate against at different name, then there is the risk that this might count as a "transfer" of users. There is a yearly limit on the number of times you can transfer users.
In theory, Student and Home accounts cannot be "transferred" at all, so there is a risk that it will be denied. If that happens, you need to contact Mathworks Support.
The transfer limit and the limitations on transers are the reason that I say that it is far easier to just change your username back.
