The answer to 'why is my sink no hot water' is that the most common causes are a faulty water heater, a clogged or blocked hot water line, or a problem with the hot water valve or faucet. Symptoms may include only cold water coming from the faucet, the water heater not heating up, or a lack of hot water throughout the home.
To troubleshoot, first check the water heater to ensure it is powered on and set to the proper temperature. If the water heater is working, inspect the hot water line for any blockages or kinks that may be restricting flow. You can also try turning on other hot water taps in the home to see if the issue is isolated to just the sink. If the problem persists, the issue may be with the hot water valve or faucet itself, which may need repair or replacement.