Underscore When trying to enter BIOS

Basically i got this new motherboard with the cpu.
I put everything together with the stock cooler and the first time i fired it up the temperature of the cpu went up to 90C right off the bat when i started it up.I checked this on the bios.The computer shut down by itself to avoid a fried cpu.
To avoid this i bought a custom cooler and installed that on the motherboard.It seems to be doing a pretty good job since its warm and that means it sharing the heat with the cpu.
The problem is now i cant get into my bios.whenever i try to go into the bios a flashing underscore appears on the top left corner of the screen and wont let me in the bios.
I need to get in the bios and tweak some settings because my pc wont boot into windows.A blank screen will appear after the verifying DMI pool data and do nothing, so i have im trying to get into the bios to see if i can fix it.
Its strange because i could get into bios when my cpu was getting fried but i cant now when its cool?
You dont suppose my cpu has been damaged or anything?
Any help REALLy appreciated since im left without a computer for 2 months now.PLease help ive searched all over the net to try to find a solution for this.