Car is heating up and temperature gauge is not showing

Hi sir my car coolant is heating up. I have checked there are no leeks in its undercarriage then somebody told me that it may be due to radiator cap seal because I have not found any leak in it, but can you tell me please what may cause heating it up? Can it it be gasket or anything else? It happens twice in one day because some days ago my brother was learning on it how to drive in back roads and continues used first and second car because the road was only where we could go up to forty kph. So please kindly tell what problem you think is with car engine it is boiling water from radiator well waiting for reply. Thanks indeed.