55 year old father had sudden feeling. Of tiredness and exhaustion when. Walking up the stairs few. Days ago. The day after he felt nothing.?

Check heart. Exertional symptoms could be 'angina' or coronary heart disease. He should get seen and get an ekg and a 'stress test' to be sure. Hope he is better.
Arrythmia, CHF? He simply could have been very tired and did not sleep well. However, more significant issues need be excluded. Does he have one or more risk factors for heart disease including family hx? He should check with his pcp and see if further diagnostic tests are needed such as a stress test, ecg, bnp assay, and even an echocardiogram. It may be nothing, but it could be serious as well! Opt on prudence!