mouse companionship

Question
Okay, a friend and myself bought mice one day. (Mine's name is Minnie and her is MooMoo)  However she started changing her mind, so i kept hers in my cage for the night. Instantly the two mice bonded and were all over each other.
My friend ended up returning hers the next day. Then I read online that mice need companionship so I rushed to the store and bought another mouse, but not the same one.  
Ever since then Minnie, who once was hyper, curious, and active, is now sleeping all the time. She eats and drinks water every once in awhile.
What I was wondering however, is if Minnie could be depressed because of her bond with MooMoo? Do mice get attached to a certain individual like this?
The reason I ask this is because minnie seems to want nothing to do with Nala (my new mouse). I thought maybe she was sick so I cleaned the cage and layed a Tshirt down instead of the bedding.

Do you know what's going on with my mouse?  

Answer
Hi,

Mice do make fast friends.  Your mouse is depressed from losing her friend.  However, there's nothing you can do about it now.  Hopefully she will get over it soon and bond with the new mouse.  As long as the two mice sleep together, they get along together.  Give them time, and they will develop a friendship.

Mice should actually live in threes, so when one mouse dies, this doesn't happen.  If you get a third mouse, Minnie may perk up more quickly.

Have fun with your little mice!  They make awesome pets!

squeaks n giggles,

Natasha