A sudden drop in peak height could be caused by a few things. 

If  all peaks lower than usual (including the solvent front peak), this could either indicate:

-A problem with the working electrode.  Typically, a residue can build up over time which inhibits a good signal.  Try wiping with acetone, if that doesn't help it might be time to replace it.

-Confirm the applied voltage is correct on the HTEC display.  

-A problem with the reference electrode. We recommend replacing about once a year.

-Not enough sample was injected.  Are you using an autosampler? If so, double check the injection volume in the software sequence file as well as inspect the well visually as well as measure how much is left with a pipette. (example, if you load 20 µL into a well and do a 10 µL injection, there should be 10 µL remaining in the well).  Try injecting a standard using a manual injector to see if there is any change in peak height.  

If all other peaks are the same height as usual, there could be an issue with:

-The standard/sample itself (ex. degradation/contamination)

-Contamination in the instrument/tubing