Assuming that you are booting from a floppy, you need to format the C: drive
and install an operating system on it.

If you are booting directly from the hard drive, then the MBR is messed up.
Get a boot floppy and type SYS C: to fix things up.

Boot to an emergency startup floppy and use it to investigate the state of
the C: drive. Try some simple commands like
DIR C:
TYPE C:\AUTOEXEC.BAT
to confirm that the drive is accessible.  If these commands fail then there
is probably a failure in the hard drive system.  It could be a simple BIOS
configuration problem or a loose cable or just a damaged boot file. Or it
could be a more serious hardware failure. Disk diagnostic software available
at the www site of the disk drive manufacturer will help to isolate a
hardware problem, or confirm that the hardware is OK. Most importantly, do
not try any repair procedures until you are satisfied that the disk is
functioning properly, as this is liable to make things worse.