Here’s some things to check:
1. You need an API key. If you don’t have one, see the FAQ.
2. The most common problem is that another plugin (or the theme) has loaded the Maps API twice. Google doesn’t allow that and it causes random bugs.
Try deactivating ALL other plugins and switching to a standard theme like 2015 (for a moment). If the map is working in this state, reactivate one by one to find the problem.
3. Try clearing the browser cache