My computer turns on and shows display but not booting

Hello,
I am having the mother board of ASUS company with amd processor and i am having a serious problem in starting my computer. My system was working fine before 3 months but one day it suddenly turned off from that time it is not booting regularly the sometimes it will boot and some times not but when it does not boot it just show that starting page of bios like American Megatrends and its version but it will not show any boot menu and it does not even initialize keyboard and mouse and i need to switch it off directly so i have troubleshooted for all the test cases so please suggest a way to get out of it