Have you tried upgrading to the latest version of Flash?
Due to advances in Flash exploitations, Adobe has been releasing emergency patches for Flash like they are going out of style. Also, the latest versions of Chrome may have taken the proactive step of disabling older versions of Flash.
Are you seeing a protective shield next to your URL (end of the bar where you type an URL). If yes click on it and turn it off.
Let me know if it helps.
@Minbie - can you please post the URL where this is happening? There are a few things that could be going on here. First, is the snippet on the page? It could be a flash thing like @JDahlinANF mentioned, or it could also be some CSS issues.
For reference, I did look at https://minbie.com/ and it looks like you do not have the snippet on the page. When the snippet is not on the page, Optimizely loads your page in the editor via a proxy. This works fine in most scenarios, but sometimes it can cause display or editing issues. The best next step here would be for you to add the snippet to the page. Once you do that, you should be able to edit the page as expected.
Let me know if this is helpful.