从 16.04 到 18.04 的升级过程未启动

Question

当我尝试从 ubuntu 16.04 升级到 18.04（使用do-release-upgrade）时，升级过程未启动。我得到以下输出：

Checking for a new Ubuntu release
Get:1 Upgrade tool signature [819 B]                                           
Get:2 Upgrade tool [1'260 kB]                                                  
Fetched 1'261 kB in 0s (0 B/s)                                                 
authenticate 'bionic.tar.gz' against 'bionic.tar.gz.gpg' 
extracting 'bionic.tar.gz'

Reading cache

Checking package manager
Reading package lists... Done
Building dependency tree        
Reading state information... Done
Hit http://archive.ubuntu.com/ubuntu xenial InRelease                          
Fetched 0 B in 0s (0 B/s)                                                      
Reading package lists... Done    
Building dependency tree          
Reading state information... Done

Restoring original system state

Aborting
Reading package lists... Done    
Building dependency tree          
Reading state information... Done

这里出了什么问题？我该如何解决？谢谢你的帮助。