[Flyover tool not working]

I did thank you so much! 

 

THANK YOU! 

[Flyover tool not working]

I have a 3 storey plan created in vector and ive just opened my file and when I go from top plan to flyover tool in OpenGL the screen goes blank. As if nothing that I've built is there. 

 

I've opened a fresh new file - built a wall and when I use flyover tool it works and you can see it - so it must be something in my file....

 

Please help!