It occurs when your sampling frequency is too low. The frequency components centered at multiples of your sampling frequency will overlap. Your sampling frequency needs to be higher than the Nyquist frequency, which is twice the highest frequency component of your signal.
when sampling frequency is less than twice the highest freq.component in message signal, spectral components will overlap and hence recovered message signal will be distorted.In simple terms, the no.of samples to define the message signal would be less for this condition and obviously one cannot recover the entire message signal.
The Nyquist frequency is minimum sample frequency to detect the presence of a signal; is to sample the signal at least once in the postive and in the neagtive amplitude time slots. The sampling frequency is thus 2 x the frequency of the sampled signal. The Nyquist frequency only guarantees the detection of sampled signal. It does not guarantees amplitude reconstruction