Disk Error after reseating RAM

[Apollo Serenus]

I recently received a refurbished HP Compaq 8200 a word processing PC separate from my gaming system. I noticed it was extremely sluggish and quickly ascertained only one of two RAM sticks was recognized by the system. Recent vids led me to conclude a RAM stick came loose in shipping.

Turned off system, waited two minutes, opened small form case, pivot up hard drive enclosure, reseat RAM, pivot hard drive enclosures back in place, system fails to boot up. Where I should get the Windows Loading screen, I get the message:

"Non-System disk error or disk error

replace and strike any key when ready"

 

I check the BIOS and my hard drives are not being recognized. My initial thought was a cable came loose, so I reseated all cables relating to the hard drives. No dice.

 

I am out of ideas as to other possible causes.