Start by getting all the info you have e.g.
- did she make an itinery of where she would be from day to day (ie where in Italy)
- did she tell you any of the details of the hotels she would be staying in
- is she planning on visiting any friends contacts there
Collect as much contact info as you can from the above, call hotels and explain the situation. I used to work in a hotel when I was a student and they will be more than happy to help to do whatever they can to contact her if she is staying at their hotel
Leave messages with the friends (if any) she is visiting.
Also contact the US consolate and ask them for help as well and they might be able to offer other suggestions. As far as I know (and hopefully Conley will read this and can help as well as he lives in Italy and knows much more than me), tourists have to register on arrival and maybe the US consolate can find out where she is staying from this registration. Here are the contact details with the consolate in Milan (there are 3 in the country and not knowing where in Italy she is staying I picked this one http://milan.usconsulate.gov/acs.asp ):
EDIT - Thanks Conley, good to get your input as normal :0)