Thumbnail does not work

I have problems getting the featured image thumbnail to show. It did work when I used a square 200×200 image from google, however I dont know if thats the problem. When I use inspect element in my browser it shows this with the image that doesnt work

<img src="" alt="MY POST TITLE">

While it show <img src=”” alt=”www.MYIMAGELINK.com”> and it does work, hopefully you guys can help me out. Thanks in advance.

Note: I’ve tried googling it but couldn’t figure out an answer, I was guided to the functions.php file but I don’t know how to solve the problem.