Hi Anna,
I am sorry to hear about your situation. It is not cool to loose your passport. I lost mine too once. But, the most important thing is of course that you get your new passport back. With regards to the
visa, I would contact the USCIS. Check on www.uscis.gov and try to call any office in the area you are and ask who to be referred to. You may also want to talk to your embassy. If you had other visas in your passport too, then you need to contact the government offices in those countries. I do unfortunately think this will take some time, but it is never convenient to loose your passport.
Good luck.
S.L.