First establish whether the loss affects every fixture and what kind of water-heating equipment is present.
The short answer
Check whether hot flow is absent, merely cold, or becomes cold unusually fast. Record the equipment type and any visible error code without opening panels. Keep gas, electricity, combustion, and pressurized-equipment work outside a household troubleshooting experiment.
What to notice
Separate delivery from temperature
A hot tap with no flow is a different symptom from normal flow at cold temperature. Compare another fixture and note whether only one mixing valve is affected. A whole-house temperature problem can involve the heater or distribution, while one fixture can have a local issue. Neither pattern is a final diagnosis.
Capture equipment facts safely
Photograph the exterior model and serial label if it is accessible without entering a hazardous area. Note tank or tankless type, energy source, indicator lights, display codes, unusual sound, visible water, and recent service. Do not reset a device repeatedly or relight equipment contrary to its instructions.
Ask what specialty is needed
Explain the fuel or power type so the recipient can confirm the right qualification. Some issues involve plumbing, gas fitting, electrical work, controls, or appliance service. Ask who is responsible for each. A visit should distinguish diagnosis from a quoted replacement and identify any permit or inspection responsibility.
One useful question
Should I raise the thermostat to test the heater?
Not as a general test. Hot water can cause scalding, and the equipment may have a separate fault. Preserve the current setting and symptoms for the provider. Follow manufacturer instructions and qualified direction for any adjustment.