I have v3-771g and its shows bootmgr error and all boot options have disappeared

if your system has windows 8.x

First of all try to check if ALT+F10 at boot take you to windows 8.x recovery and you can reach command prompt from there:
troubleshoot-advanced options- command prompt (if this works, go to point 4, if not point 1)

1. create a vanilla Windows 8.1 installation media from this link:
http://windows.microsoft.com/en-us/windows-8/create-reset-refresh-media

2. Once you boot using the Windows installation disc, on the Windows 8.x installation window, click on Repair your computer.

3. Click on Troubleshoot and in the Advanced options. First thing you may try is the Automatic Repair.

4. If that does not help, then select Command Prompt. Enter the following four commands into the prompt.
bootrec /fixmbr
bootrec /fixboot
bootrec /scanos
bootrec /rebuildbcd
Once the command completes, restart the computer.

you can try the same thing if you have a bootable windows 7 media or if ALT+F10 works and can open a command prompt.

or you can download a free tool, like minitool partition wizard and create a bootable media and then choose to fix MBR.

https://www.partitionwizard.com/partitionmagic/how-to-fix-mbr-without-windows-7-dvd.html