Monodroid problem

Hello,

First of all I am a student and new since tomorrow to xamarin.

Earlier today I build a first demo app and it run perfectly on my htc desire. Now when I try make a new project and run it, it doesn't show my phone and when I close it it gives the error:

**MonoDroid does not support running the previous version. Please ensure your solution builds before running or debugging it. **

Any suggestions?

Greetings Jochem