Just had this problem recently. I was reorganising my WordPress themes file structure (clearing a huge mess off my desktop basically), and then BAM! MAMP won’t start up apache, what a nightmare!

Well anyway, if you google this then there’s plenty of reasons why this could happen, OSX conflicts etc. The issue I was having was so basic I almost missed it;

If your localhost (your first server, the one you can’t remove) MAMP PRO server is looking in an invalid location, Apache will fail.

Yep, it’s that easy really, I’ve got about 20 – 30 servers running in MAMP PRO at any given time, so when I moved my file structure, each one of them failed, no issue I thought, and it isn’t, as long as your base server is looking in a valid location. So hopefully this will save someone a little time that cost me!