这是一个非常常见的问题，但请耐心等待，因为我已经尝试了您提出的所有建议。因此，我得到的结果如下：
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 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.
所以我尝试安装unity-control-center
Depends: unity-settings-daemon but it is not going to be installed
所以我尝试安装unity-settings-daemon
Depends: gnome-settings-daemon-schemas (< 3.10) but 3.12.2-0ubuntu1~trusty2 is to be installed
根本问题似乎是gnome-settings-daemon-schemas需要 3.10 或更低版本，但由于某种原因无法获取。
因此，您可能会考虑尝试以下操作：
sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
sudo apt-get clean
sudo apt-get autoclean
sudo dpkg --configure -a
sudo -f
我已经登录到 gnome，当我尝试启动lightgdm系统时，由于出现了一些有关打印机后台处理的事情，系统立即挂起。
我目前有 gdm、kdm 和 lightgdm。
我想让我的电脑恢复到 gnome 和 ubuntu unity。有什么想法吗？（重新安装是不可能的）
答案1
Unity 基于 gnome。许多 unity 软件包（和ubuntu-desktop）都依赖于 gnome 软件包。
因此这里的问题似乎是添加了 Gnome 3.12 ppa。
gnome-settings-daemon-schemas
是 Gnome 的一部分。您尝试安装的软件包 ( ubuntu-desktop) 需要来自 gnome 的 3.10 版本，但它无法获取 3.10 版本，因为 ppa 指示它从 3.12 存储库获取。
如果你想要恢复 Unity，你需要安装以前的版本：
sudo apt-get install gnome-settings-daemon-schemas=3.8.6.1-0ubuntu11.2
但这会因为依赖关系而删除一些 - 因此 gnome 3.12 将会中断......
The following packages will be REMOVED
gdm gnome-control-center gnome-session gnome-settings-daemon gnome-shell
The following packages will be DOWNGRADED:
gnome-settings-daemon-schemas