Using iGPU without graphic card and monitor..!

hi every body..
Newly I build a system..
i5 3570 k,
P8z77-vlx
Radeon HD 7870
But yesterday I get my graphic card to warrenty because of some problems..
Now it take a week for them to change my HD 7870 but now i want to use my computer without that..for some my simple works..
But when I attach my monitor to motherbord(hdmi,disub,dvi) it does'nt show any thing..
I mean that..like monitor is completly off..or no signal mode...
But i found that the system is up corectly..
I mean I found the OS is going up and...
How can I activate my iGPU in this situation?
I can't even access to bios..cause I don't see anything..