Please contact an attorney directly in the area where you were arrested. California's gun laws are complex and fact-driven. The legality of the loaded weapon will likely be dependent on the scope of your permitted carry terms, so being whatever permit that you may have when you speak to the attorney. The severity of the crime for which you can be charged depends in part on the lawfulness of your possession of the firearm. While a misdemeanor conviction for carrying loaded and/or concealed does not preclude you from possessing a firearm in the future, it could have major impacts on you in other ways.
Any statements I make in these forums (fora?) should not be taken as direct legal advice, merely informed guidance. This is true due to the anonymous nature of this venue, and the incomplete information which is invariably provided by the questions. It is imperative that you consult directly with an attorney regarding your specific situation before acting on or relying on anything represented here. Period.
As Mr. Moore explained, you need an attorney immediately. Go meet with a few and hire the one you feel most comfortable with. Ideally you want one that knows California firearm laws but also the court where your case is being handled. It sounds to me like you have a "guard card" which allows you to carry your loaded handgun, but only if carried openly and during the scope of your employment. If you were not on duty and the gun was loaded, you have a problem. Do NOT post anymore facts here about this matter, and do not discuss what happened with anyone other than a criminal defense attorney in a confidential setting. A conviction could definitely impact your future employment. If you cannot afford a private attorney, then request a public defender at your first court date.
Michel & Associates, PC
All my comments here are intended for general legal purposes. None of my comments here establish an attorney-client relationship with anyone. None of my comments should be relied on in taking legal action without first consulting an attorney.