My laptop was dropped and it will no longer boot

I have a Toshiba A45-S121 laptop that I dropped off a coffee table onto a floor covered with carpet. It will no longer boot. When I power it up, the backlight goes on, the fan goes on, and I see the HD light blink a bit. Nothing else – no video, no HD activity, no beeps. I took the cover assembly off and reseated all the flex cables, and tried another HD I had. Still nothing. I think the motherboard is dead.

Why do you think the laptop has a dead motherboard? Because it will not boot up?
When I repair dropped laptops, first of all I reseat all connectors inside and very often it fixes the problem. I hope you didn’t crack the screen.
Try to reseat the memory module (by your description it could be a dislocated memory) and see if it fixes the problem. If not, reseat the wireless card. If it doesn’t help, I would try to reseat the CPU. Here’s a guide for taking apart Toshiba Satellite A45 laptop.
The memory is located under the keyboard, but it’s very easy to access it on this model. Follow steps 8-11. It’s not necessary to disconnect the keyboard. Remove the battery and disconnect the AC adapter when you work on the laptop.