If this message suddenly shows up, one of two things.
A) Motherboard issue
B) Hard drive died
Before doing anything else, turn the system off, unplug the power cord to the PC itself and hold in the power button for 30 seconds. Let go, plug it in again and turn the system on. Does it boot up? If not, grab
Download Hiren and burn it to CD / make a bootable flash drive
Boot your system to it, and go to hard drive tools. From there, you may have to go down a couple levels in the menu to find it, but run GWSCAN and see if it detects your hard drive. If it doesn't, then the system may not see it either. If it does see it, run a full extended test on the drive (it'll first run a quick test) then if it says it repaired errors, try to boot up again. If not, let us know the error message.
Alternatively, it may be an issue with the memory or something with the motherboard. You can use the same boot CD to run a Memtest to see if the RAM in your system is OK.