Which disc are you trying to boot from? The Mac Mini 2010 will only boot from a disc that is OS X 10.6.4 Snow Leopard or later? If so then try the steps below. Reset PRAM Hold Apple+Option+P+R keys at startup Reset SMC. Shut down the computer. Unplug the computer's power cord. Wait fifteen seconds. Attach the computer's power cord. Wait five seconds, then press the power button to turn on the computer. Update: Is that the OEM disc you received with the machine? If not that is why it wont boot. You could put the Mini hard drive in another machine that can boot to 10.6.3 install the OS and update it and put it back in the mini and it will work (If the OS version is the issue.) Update: I just saw your response above regarding USB. Can you not get to startup manager? Hold Option key at startup and you should see the mouse pointer on the screen if no bootable drives are present.
FYI My S400 would not start sometimes. If I banged it in the palm of my hand occasionally it would then work. I was also having terrible trouble with batteries not working and showing low charge when they had been charged.. I opened it up as a last resort. On the front left side below the shutter button there was a small ribbon cable that under a magnifying glass I could see was slightly out of line. I reseated it and the camera has acted flawlessly since.
I've already spent 30 mins trying to remove these screws. DO NOT attempt this unless you have plenty of time to waste. Just take the top case/keyboard off and remove the screen assembly to save yourself a big headache. Thanks for the great site.