Hi all - same problems with my A300 last night - it was running hot, and the fan noise was annoying me, so I turned down the power settings to "Power Saver". Then things started going wrong with USB ports at first, nothing recognised, etc. I shrugged it off and then turned off the computer for the night. This morning - black screen of death on boot up, no hard drive activity, etc. Occiasionally made it to the bios screen but that didn't seem to help. Removed the RAM and put it back, all to no avail.
What did work however was a bit weird. I tried rebooting with a USB hard drive connected, and after a few attempts it worked! Still with major problems - only half the RAM recognised, no usb ports working, no fingerprint logon, blah. So I kept it running for a while, changed the power setting back to balanced, ran a virus check (negative), backed up my files, ran a diagnostic (passed with flying colours depsite the fact that half the systems didn't work :)) and checked the disk, and now... running like a dream again.
For most of you, this problem is probabably something to do with power settings. If you can't get past the black sceen try giving it a bit of a jolt by plugging something into the USB ports. You never know, it might just work.
Just have to hope that this doesn't come back...