Hi, i never had a problem to write a riff/melody/instrumental song that reflects an emotion, and make myself satisfied with it. But when it comes to lyrics im completely blank... I can have a clear idea of a theme etc, and be inspired, but nothing comes to mind... Any tips?
Well I don't know what you've done so far, and I've only just started trying to write songs, but the best thing I've done so far is just to brainstorm. Some people make mindmaps and that sort of things, others just write down anything in their head.
The most important part for me is the editing. Write down as much as you want/can about what's going on in your head and then when you come back to it you can be critical.

Other than that, I find being in a certain mood helps me. Sometimes you just get writers block and you have to stop and leave it.

Or you can get drunk. Ernest Hemingway said "Write drunk; edit sober"
i tried putting an image of what I want to write about in my head, keeping it clear and focused, and just writing words to describe it, and worrying about the order and rhythm after I have a general idea on what i'm doing.
Do you write lyrics a lot? It's just like playing guitar. The more you practice, the better you'll be at coming up with awesome lyrics.

There's a nice writing exercise. Put a timer on 5 minutes. Put your pen to the paper and write for those whole five minutes without stopping for any reason. Even if the words are "I don't know what to write but I'm trying." You have to write for five minutes. Eventually, your mind starts to mesh with your pen and cool stuff starts happening.

Just practice. Literally, practice writing lyrics and it'll get easier to express yourself.