Your first steps with no hot water
If you have no hot water, report it to your landlord in writing right away and treat it as urgent, since hot water is a basic requirement for a livable rental in California. Note the date it stopped and keep copies.
A complete loss of hot water affects bathing, cleaning, cooking, and daily life, so it should be repaired quickly — not left for days. The cause is usually a broken water heater or a plumbing failure, both the landlord's responsibility. Put your request in writing, such as a text or email you save, so there is a record of when the landlord was notified.
If it drags on, keep a running note of how long you have gone without hot water, since that matters if the problem is not fixed promptly. If your landlord will not restore hot water, a free review can help you understand your options.
This is general information about California tenant rights, not legal advice. Every situation is different.