Yeah that sounds like a real problem. There's no way anyone can help you though. Not without seeing your source code, anyway. ;-)

I don't want to sound like I'm being dismissive, but could it be just that you have managed to break all of your projects so far? Can you give more detail about what you mean by 'doesn't start'? When do they stop starting - is it always after you've edited the code?

ok, sry. It shows the normal greenfoot load screen, at this points he stops. I let my pc two hours to open every of those projects (not at the same time) and there was still the load screen.

First of all, make sure you're only running Greenfoot once - that is, if you are running it already, don't start it again. Ideally keep only one project open at a time. Close any other running programs that you don't need. If it keeps happening, find your debug log file (greenfoot-debuglog.txt) and send it along with all the other required information to the Greenfoot support team, see details here.

I know this discussion is old, but the problem still exists and I think that the project.greenfoot file is broken, cause when I replace it with an another project.greenfoot file (from an another scenario). I read the debuglog, but everytime there isn't something for this start of greenfoot

I know this problem, but I unfortunately don't know how to stop it. In my project (a realy, realy big one) there were so many classes that it didn't start anymore. I don't know why but as I found a way to remove some classes (project still worked) this problem no longer exists. But in my case it were more than 100 classes.