Q: Cleaned inside of MacBook, rebooted to black screen

I'm in the process of selling my mid-2009 MacBook Pro. I live in a dry, dusty climate so I thought id open it up and give it a cleaning Which I havent done in 7 months since I put in a new battery and hard drive. Instead of canned air I just used my bulb air blower that I use to clean my camera lenses. Blew some dust out of the fans, the vents, and over the top top layer of everything inside. I made sure not to touch anything. Put the bottom plate back on and when I powered it on I'm just getting a black screen. I get the start chime and I can hear that it's on.

Did some reach and reset the management controller by pressing control+option+shift+power at the same time and then rebooting, but that didn't work.

Next tried a key-pass sequence by pressing the power button once (supposedly bringing up a dialogue box I can't see), pressing S (sleep shortcut key), held down the power button to force a hard shut down, waited at least 15 seconds, and pressed the power button again. Same black screen.

Lastly I tried a PRAM reset - pressing down the power button and when hearing the chime holding down command+option+p+r and waiting for another chime before releasing. Read on a different site to let it chime twice before letting go. Neither worked.

Don't know where to go from here. I was careful when I blew dust out and didn't touch or move anything, except for running a microfiber cloth over the top of the battery. Im supposed to meet someone in a few hours who potentially wants to buy this but I obviously can't show it to him with only a black screen. The local Mac tech doesnt work on Sunday's. Any suggestions as to what happened or what to try next?