Kernal Building

Hi i am new to the topic of Building Custom Kernels and i am using ubuntu 12.04. Recently i build the latest kernel 3.15.0-rc by following the below tutorial http://kernelnewbies.org/KernelBuild at first it run correctly after that again i build the same kernel at this time i don't what did i done wrongly it is not working properly .when i am starting my system it gives the error like "The System is Running in low graphics mode you have to set manually configure it" and i tried with grub previous linux version .it is also gives the same error. So can any one please help me how to fix the issue or what did i done wrong? please suggest me any tutorial which will explain what happened internally when building custom kernels? Thank you