Help Macbook Air Grey Screen on Boot

Hello,
I rebooted my macbook air last night and upon reboot, I just get the grey/white screen (no apple ever shows up). I can't find my Macbook Air startup disk (it may not have come with one), but I attached a superdrive and put in a startup disk from my iMac to see if that would work (and pressed the Option key upon restart) and nothing happened.

Now I'm stuck. Any advice on next steps? Thanks.

FYI - I was running a beta version of Yosemite

I tried resetting the PRAM (option+command+p+r) and that worked, but when I hold down the alt/option key on reboot, nothing happens - I just get the white/grey screen (I had a USB drive loaded with recovery assistant).

I also tried command-r and nothing happens.

As a side note, it only appears that the USB port on the right hand side of the keyboard lights up my USB drive. The other side (by the power cable) does not light up my USB drive.