Xcode is one stop shop development app for OSX & iOS devices including all Macintosh Desktops & Laptop, iPhone, iPad & iPod touch.
Note: I Assume you are on OSX Yosemite 10.10.3 or Latest Operating System & Xcode 6 or later installed.
With release of Xcode 6 & later while developing & running your iOS app in iOS Simulator on your Mac you face with this bug something like this ‘Unable to boot iOS Simulator’ with blank screen.
Here is how to fix it
Select iOS Simulator then go to top Menubar > iOS Simulator > Reset Content and Settings > Reset (On command prompt press reset)
This function will delete all data entered by you & it will reset selected iOS Simulator device to it’s original factory setting for iOS Simulator.
If Solution 01 doesn’t work then in way it means your Xcode installation is buggy or it is installed with errors.
Step 01: Delete Xcode from your Application folder
Step 02: Empty your trash
Step 03: Reboot your system
Step 04: then with help of ‘Disk Utility’ verify your Macintosh HD for Disk Permission & then Repair your Disk Permissions for your selected Macintosh HD
Step 05: Now from Mac App Store install Xcode. With fresh installation of Xcode your problem should get fixed with itself.
Hope it helps,
Thanks & Regards