I can never think of any funny web comic ideas!

I'm not entirely where I could post this question, but I think this is pretty appropriate.

For the past couple days I've sat down and tried to think of something funny to put in like, a 4 panel webcomic, but I can never think of anything, and then when I think too long, trying to get the job done, I get upset and frustrated.

Anybody have any advice? I've done a few comics in the past which I like, but I want to start a running webcomic but I can't if I keep feeling like this every time I try to think of an idea... I have some ideas, but right now I don't think there very funny, I can almost justify that by realising that a lot of my favorite comic makers have a couple comics that I don't find very funny either. But the thing is they can get away with it as they already have an established audience, I don't and I need to make a good first impression!

Any help would be greatly appreciated!