我尝试了几次讨论，然后决定在这里提问。我不知道什么时候失去了 Unity 桌面。我不得不依靠我与 Unity 一起使用的 Gnome 和 Cinnamon。当我使用其中一个替代方案时，我注意到 Unity 桌面不再是登录时的选项，并且消失了。大多数在线指南都说使用
sudo apt-get install ubuntu-desktop
但我在这里遇到了依赖问题。
另一件事是我想升级到 16.04。我曾想过等到发布时再升级，但现在无论如何都会继续升级。但为此，当我使用
sudo update-manager -d
我收到了部分升级通知，最终升级失败。当我同意部分升级Ubuntu 会说impossible to install ubuntu-desktop或类似的话。部分升级抛出错误并返回部分升级选项。因此，我也无法尝试 16.04。请帮忙。
$ sudo apt-get install ubuntu-desktop Reading package lists... Done
Building dependency tree Reading state information... Done Some
packages could not be installed. This may mean that you have requested
an impossible situation or if you are using the unstable distribution
that some required packages have not yet been created or been moved
out of Incoming. The following information may help to resolve the
situation:
The following packages have unmet dependencies: ubuntu-desktop :
Depends: ubuntu-session but it is not going to be installed
Depends: unity-control-center but it is not going to be installed
Depends: unity-settings-daemon but it is not going to be installed
Recommends: xul-ext-webaccounts but it is not going to be installed
E: Unable to correct problems, you have held broken packages.