If the only thing on the screen is the flashing cursor, disconnect EVERYTHING except the monitor - USB devices (inc keyboard and mouse), drives (hdd and cd). Then try powering it up again. It's not unknown for machines to refuse to pass the POST if a USB device won't respond to a reset command, usually USB hubs are the most common for that.

If the machine actually does a POST, IDs drives and memory etc, then there should be nothing to stop it booting. Failing that, aim for the CMOS reset jumper/button (again with no devices connected, then try booting it.