If it's the hot tap, turn it on and leave it on until the whole body of the tap is good and hot. If it's not, as it appears in this case, trickle some boiling water over it from a kettle. Try one of those rubber or plastic devices made for undoing bottle or jar tops. Make a discreet mark with a pen or pencil or put a sticky label on the top to give you some way of seeing if the top is actually turning. As you don't know(?) which way the thread turns, if there is a thread, some close attention will be needed to see which way you get "give" as you work at it; from which the direction can hopefully be deduced. Working back and forth is usually a good start until the direction of the thread becomes clear. Hold onto the tap handle as you attempt to turn off the cap, don't simply force it against its end-stop.
If all else fails contact the manufacturer or google the name via an image search until you find the same model and hopefully instructions on how to proceed.