我的是因为磁盘空间不足造成的。
解决方法如下：
ok i tried everything on this page, but the fix for me was to make some more room. "df -h" showed sda1 as 100% so then i run "du / | sort -g" and found trash was like 30gig... 80% of harddrive, so i did "rm -fr ~/user/.blah/trash" and followed up with another df -h showing 14%, so a final reboot and i was back in.
或者自己进入目录，删除一些不必要的文件。
参考：https://askubuntu.com/questions/141606/how-to-fix-the-system-is-running-in-low-graphics-mode-error