It's a new Inspiron (bought 5 months go)

Sometimes display just gets black. I thought it was a creensaver or somehing, but lap top is working with black screen and only way to return it to normal -- switch off. Now it happens not so much but I am afriad it could get worse. What should I do?

 

I didn't buy any additional service so I guess 90-day's warranty was only a had and now it is gone. How much they will take to fix it?