问题描述：电脑安装的的win7+Ubuntu1404。今天上午Ubuntu提示升级后点同意后更新，关机后再开机，显示windows启动管理器，默认情况是进入管理菜单中的Ubuntu，然后显示grub启动菜单，选择grub菜单中的Ubuntu后结果电脑一闪而过后又重新启动了。现在win7能正常启动，而Ubuntu不能启动了，我现在也看不grub菜单了，不知道什么原因，向各位高手求救。

使用easybcd显示引导菜单信息如下：
Windows Boot Manager
--------------------
identifier {9dea862c-5cdd-4e70-acc1-f32b344d4795}
device partition=C:
description Windows Boot Manager
locale zh-CN
inherit {7ea2e1ac-2e61-4728-aaa3-896d9d0a9f0e}
default {6815cd4c-aaee-11e1-ba02-ebe843062588}
resumeobject {6815cd45-aaee-11e1-ba02-ebe843062588}
displayorder {6815cd46-aaee-11e1-ba02-ebe843062588}
{6815cd4c-aaee-11e1-ba02-ebe843062588}
toolsdisplayorder {b2721d73-1db4-4c62-bf78-c548a880142d}
timeout 5
displaybootmenu Yes

Windows Boot Loader
-------------------
identifier {6815cd46-aaee-11e1-ba02-ebe843062588}
device partition=C:
path \Windows\system32\winload.exe
description Windows 7
locale zh-CN
inherit {6efb52bf-1766-41db-a6b3-0ee5eff72bd7}
recoverysequence {6815cd49-aaee-11e1-ba02-ebe843062588}
recoveryenabled Yes
osdevice partition=C:
systemroot \Windows
resumeobject {6815cd45-aaee-11e1-ba02-ebe843062588}
nx OptIn

Real-mode Boot Sector
---------------------
identifier {6815cd4c-aaee-11e1-ba02-ebe843062588}
device partition=C:
path \NST\nst_linux.mbr
description ubuntu