Dropped a Dell All in One hard to the floor and now when I turn it on it says

Try to open up the system, unplug and replugin everything. Remove the video card, and the small back cover over the blue panel - there's a VGA connector for the onboard video card - test to see if that works. If not, go for a minimal setup (one stick of ram and power) to see what it does in order to troubleshoot it.

Possible reasons for a system not booting after one has dropped it, in order of least likely to most likely:

The motherboard was damaged due to impact. Items to suspect from the drop in order are: RAM, CPU or case fan, expansion/graphics card, SATA cable, or possibly a jumper on the motherboard may have gotten loose. The following is what I suspect:

The All in One was damaged due to stress induced by attached cord pulling (i.e. VGA socket may be damaged if the fall caused a cable to pull on it too hard).

The all in Ome worked loose due to impact and isn't fully making contact with board socket, or board socket has been damaged.

The All in One was damaged due to impact. Items to suspect in order are: hard drive, daughterboard component along side containing USB or audio ports, power switch, power supply, motherboard or plugin-component such as RAM or motherboard.

Try to troubleshoot and diagnose it yourself to save money. If not you'll have to take it in.