Can't open exported file on FCP

Hello There,

I need urgent help, I've had FCP on my computer for a while and decided to try using it. I made a big project that took me a while is is about 40 mins long and just as i'm about to finish it, it wont let me open the file.

When I tried exporting to a file it pretty much worked instantly and went straight on my desktop but when I tried opening it, Quicktime saying "Quicktime Player cant open (file name)"

Someone please help me I need to finish this project asap.

Im using a Macbook Air and final cut pro version 10.1.4