So i've hooked up my PS3 to a friends HDTV with an HDMI cable to test it out but now i'm going back to the old SDTV but the screen won't show up when i turn the system on. How do i change the video settings back on the system back to SDTV?

You have to go to system setting then video settings I believe. I could be wrong but its worth trying.

I think you just need to turn it off then on again... It needs to re-set itself to the SDTV configuration...

While in standby, hold the reset button till you hear two beeps.  That resets the video settings.

Hold the on button and wait for three beeps.

as yakuzaice said, hold it for about 7-10 seconds, there should be another beab, then it will reset the video settings

Yakuzaice is right.
If at first you don't get the screen to show up, just try it again.

Resetting the output signal is one of those features that should be more well known. It's really helpful so you you don't have to always be mindful of setting the video settings prior to moving the PS3 to another TV.

hold button until u hear beeps or go to ur friends tv and switch to avmulti out.

This was a huge pain in the butt on the PS2... I set my system up with the wrong options for component and what a pain that was...