Since the Domino CLI software is somewhat at the mercy of the user environment, both OS and Networking on the users desktop/laptop, it can fall victim to numerous failures such as improperly installed Java versions/improperly configured proxies/out of date certificates.
That said, the first rule of CLI troubleshooting is to make sure you have downloaded and installed the latest version of the CLI.
CLI versions are tied to the version of your Domino deployment and are very often neither forward nor backwards compatible so always make sure you have the latest version from your deployment. It will likely be one of the first things Domino Support will have you verify regardless.
The second rule?? We have numerous KBs on this topic, so look through the available KB articles to see if your specific failure is already documented.