This can happen if a bootable but non-working CD is in the drive or a USB device is plugged in and the system is seeking to boot from those devices before the Hard Disk.
Remove all add-on devices and try again - if that fails, check in BIOS>Boot Order that the hard disk is top of the list.
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Without having one of those, it's hard to say which key will open BIOS menus but the Delete key is favourite. Did you try audiovoodoo's three suggestions?
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Having had a quick search it looks like Acer tends to use either 'F1' or 'F2' to enter the BIOS screen. Once in the BIOS you will need to set the boot order so that SATA HD is set to be the first boot device.
You need to go through the menu options that you can see there. Normally you can use the arror keys or page up / down keys to do this. You're looking for a section that says boot priority, in there is where you will need to make the change.
Does any hard disk show up on the main first BIOS menu page that you see? If not, the drop may have shaken it loose and it needs to be put back. There may be a hatch on the base held down with two screws and with an icon looking a bit like a barrel printed on it. Take that off carefully and ease the hard disk back into place.
If it looks any more complicated than that, post back with the make and model number.
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