notoriousmib

I've not long got my car back from the garage after having another engine dropped in but thats another story . It's been back for 4 days and been runnin well sweet never missed a beat untill sat night when coming home was slow on the start took a bit to go. She started and off i went never missed a beat all the way home. Went to go out today though and it wont start at all. Plenty of life in battery and starter is ok but it just wont fire up. As turning the key it's just letting out a sort of putt putt putt noise but just wont fire. Like i say just decided to do it out the blue i have no idea what it is? Anyone any ideas? will be taking it back to get sorted unless sowt i can do myself. Help me all wise ones
Ta.