fiesta problem, won't start!

Can someone please help! I changed the engine in my ford fiesta 1.25L 16V zetec 'R' reg. The engine started straight away. It ran for about 10 minutes then stalled. i tried starting it again, but it wouldn't. I left it for about 15 mins and tried starting it again. It started and ran for about 5 mins this time, before stalling again. After this it never started again. what can be the problem?