Non-system disk error or disk error - technical definition

A DOS error message. The full message is

    Non-system disk or disk error
    Replace and press any key when ready

This usually means there's a non-bootable floppy in drive A. The computer looks for DOS on a floppy before it looks for DOS on the hard disk. If an ordinary floppy is in drive A at startup, it causes this error. Remove the disk and press any key.