sometimes my laptop is not turning on for few minutes & turning on after pressing the power button for like 15 sec

i purchased this laptop brand new from an authorized dealer in Sri Lanka 3 months ago. They have upgraded my RAM (4GB to 8GB ) and my storage (256SSD to 1TB SSD). Usually it is working fine but sometimes it is not turning on by pressing the power button. no fan no display no lights... nothing happens. but it turns on by pressing the power button after few minutes. or it turns on by long pressing the power button for like 15-20 sec. My System time lags every time if this happens and i have to manually reset the time. what should i do ? is there a quick fix or should i visit dell service center. it is still under warrenty