Why is my screen not working?

my screen doesnt have any physical damages. all the buttons are working but the screen is not. last time this happened after i fore restart my phone and remove and insert again the simcard it becomes okay the screen is working already. but it keeps on working and not working vice versa until now its been 3 days since i cant use my phone because of the screen. the screen is completely fine also no internal damages that i can see.