Thank you for your reply.
First off, lets get the support pages up in this thread for your Compaq mini CQ10 400CA computer.http://h10025.www1.hp.com/ewfrf/wc/product?product=4222475&cc=us&dest_page=product&dlc=en&lc=en&query=Compaq%20mini%20CQ10-400CA
I have read through this post and realize you have tried several things already, you mentioned you did a hard reset, with the battery out and the power button pressed in.
The hard drive worked in another computer, but not in this one.
Below I am offering some possibilities as to what the cause may be and how to perhaps correct it.Recover an Earlier Version of BIOS
Please read up on this in more detail at the link below.
If your notebook's BIOS becomes corrupted - either due to a bad install of a BIOS upgrade or a some other software-related mishap - then the computer displays a black screen and does not boot. It will not load any BIOS or System Recovery screens, nor will it boot from peripheral devices, because those actions require a functional BIOS. If this happens, you can recover an earlier BIOS version in order to restore your notebook's basic functionality.
(more info at this link below)http://h10025.www1.hp.com/ewfrf/wc/document?docname=c02693833&tmp_task=solveCategory&cc=us&dest_page=product&dlc=en&lc=en&product=4222475&query=Compaq%20mini%20CQ10-400CA
Please make sure your RAM stick is fully seated in its slot and secured in place.
Make sure the hard drive is making a secure connection to its electrical mating contacts.Overview of How to Troubleshoot Problems When a Computer Does Not Start Up or Boot Properly
Link: http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01443371&tmp_task=solveCategory&cc=us&dest_page=product&dlc=en&lc=en&product=4222475&query=Compaq%20mini%20CQ10-400CASystem Recovery
Link: http://h10025.www1.hp.com/ewfrf/wc/documentSubCategory?tmp_rule=39554&tmp_task=solveCategory&cc=us&dest_page=product&dlc=en&lc=en&product=4222475&query=Compaq%20mini%20CQ10-400CASystem Recovery May Fail or Display Error Message
This part answers your question on how to obtain the files needed to perform a system recovery.Overview of Recovering the OS or Reinstalling the Operating System
Performing a system recovery to return the computer to its original condition may be necessary if the operating system is damaged, the hard drive is corrupted, or the computer is infected by a virus.
Performing a system recovery removes all software and files saved to the hard drive, reinstalls the original operating system, and then configures the settings for the default programs.
HP provides several different recovery tools and processes Names for these tools include: Recovery Solution, Recovery Manager, F11, Recovery Disc, Driver and Application Recovery Disc, Roxio BackOntrack, Microsoft Backup & Restore.
for recovering notebook PCs to their original operating condition. To help you select the correct recovery process, the following table identifies the different models, the time frame when they were produced and sold, plus the originally configured operating systems. Click on the name of the document in the right column for detailed information on how to perform the recovery.
Link to downloads: http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00006110&tmp_task=solveCategory&cc=us&dest_page=product&dlc=en&lc=en&product=4222475&query=Compaq%20mini%20CQ10-400CA
Please keep me updated to your progress and the results of anything you try, you may want to try the recovery option listed in my last link for you to visit.
I wish you success and if you have any questions, please do not hesitate to ask, we are here to help you through this.