need to resolve Ubuntu 16.04 booting problem

I have recently updated my workstation from ubuntu 14 to 16.04. After this screen shows “Input Signal Out of Range, Change setting ti 1280×1028-60hz” So I to solve this I modified this command through grub menu #GRUB_GFXMODE=640×480 to GRUB_GFXMODE=1280×1028. And then while restarting I have mistakenly entered into booting menu by pressing F12. Now I cannot come out this boo menu option, I have resarted the system many times still it shows boot menu. Please help me …..boot menu page