Okay, so things don't always work the way we expect them to, but at least Maya tries to tell you why. There is a feedback window in the lower right hand corner of the interface... if this turns red be sure to read it, because maya is telling you something went wrong, If the error is long, you may also read it in the script editor. In the sample below I attempted a boolean operation with only a single object selected.

Now that your armed with the notice of what went wrong, what do you do with it? Well, if you understand the error message then correct the problem and move on. If you don't understand it, copy it or write it down, search the Maya help files or inlcude it in your post here on SM. Often, what may not be readily appearent in the panel view, is identifiable in the error text.

There are a ton of helpful people here, providing them this information will allow them to be more helpful faster.