How did my Xcode project get changed from iOS project to an OSX project?
could be 2 late to answer this question, however, this answer might help someone in the future, as the above answer wasn't helpful for me, so here is what I have done.
1- on the left top corner of Xcode click on the scheme.
2- click edit scheme
3- in the new window change "Executable" from non to the name of your project scheme "target"
4- on the top where it says "My Mac 64-bit" if it didn't changed automatically to iPhone/iPad change it manually.
5- if there wasn't any option, in bottom of the window click "Manage Schemes" and make sure the check box next to your project scheme is checked.
hope this could help and save anyone time.
By your comment you should not have an issue navigating this.
Go to your Project (in upper left) -> Project (project name in submenu) -> Build Settings -> under Architectures menu select Base SDK and choose iOS 6.1.
This should switch your project back. Now under Info you should see your Deployment Target options have switched from saying OS X Development Target to iOS Development Target
As for why it originally changed, I have no idea. But hopefully this will fix your issue.