How can I fix white lines in the screen?

I haven't tried anything because I want to know first what's the cause of this issue and if I can fix it or not.

I've got this camera during three years, and it never did present any problem.

During a time (Like, almost two years, actually), I left it in a box, because I had problems finding a good SD card.

Then, some days ago, I started to use it again, and it had this white horizontal lines in the screen.

I thought it was nothing, just a malfunction due to the poor use, that would just disappear after a few days, but today there were more lines, and then I got worried.