Mac screen goes black while in active mode

Mac screen goes black while i was using it. but i still can hear the skype sound or videos i was watching. it's like everything was okay except the screen which black. the f1 & f2 button still working, as well as the other buttons. so i decided to turn it off for like one whole day and let it rest because i thought it happens because of i use it for a too-long-time. then, i tried to turn it on, and it works! but it works only for like 20 minutes then it went black again. then, i decided to turn it off again, and now im using it for posting this and it seems to work well. but im afraid it will happen again in the future, so pleasee tell me what should i do and change. thank u so much!