The message appearing on the front panel indicate that the input is unable to read a vlaid input signal. This doesn't necessarily mean that there is a fault with the input itself. This could be that there is a problem with the signal or it could be simply that the instrumentis configured incorrectly. the first things to check are that the instrument is configured to read the type of input signal you are applying to it and that you have followed the correct wiring instructions when connecting the input signal. If all correct, measure the input signal using a DVM. Onlt after exhausting all of these factors will it be safe to assume that the input ciruit itself has failed.