With the latest AppBuilder release, there are some problems with the simulator that our developers are already working on. You can learn more and check for updates on the problems' status in this forum thread:
Issues With the Device Simulator After the AppBuilder 2.5 Update
If you believe your issue is not related to those described in the thread, please send us a sample project and we will gladly investigate the case further.
In any case in the meantime, as suggested in the thread, you should be able to use the device simulator in the in-browser client, run your app on a physical device or in the native emulators.
Please, excuse us for the inconvenience caused by these problems.