engine problem 2.5dse

hi i am a new member with a 2.5 dse bmw engine ,right i was driving around to take the wife shopping when the alternator light came on again so i thought it was time to order so off i went but on my way my rangie started to loose power and all of a sudden it powerered up again this has been doing this for a week now,the next problem i am having is that it only turns over but will not fire has anybody any ideas
thanks
si