Unable to boot PC

When attempting to boot this PC, I get the following message: “Invalid System Disk – Replace the disk and press any key”. There is no disk in the floppy or CDROM drive. When I put a bootable disk in the floppy drive, it is ignored and I get the same message. The user contends that they were on the internet and a message popped up stating “Oh you’re at it again”, then the PC locked (keyboard and mouse) so they used the on/off switch and they have been getting this message since then. What I don’t understand is why the floppy drive is no longer recognized. Any suggestion??