If you still require a visa, you'll have to see if you can get an urgent appointment at the US embassy. You should note the following two quotes from the embassy website:
1. "The Embassy does not accept walk-in applications".
Source:
http://london.usembas...w/visa/niv/apply.html
2. "Appointments are allocated on a first-call, first-serve basis. The Visa Unit receives many hundreds of requests for emergency appointments, as well as requests to expedite visa applications for business and personal reasons. Therefore we have set up and enforce criteria for what constitutes a genuine emergency or legitimate reason to expedite an application. We accommodate a large number of them. However, we can't accommodate all of them"
Source:
http://london.usembas..._visa_niv.html#niv200
You'll need to complete the visa application form:
https://ceac.state.gov/genniv/
and take a photograph meeting the relevant requirements:
http://travel.state.g...guide/guide_3888.html
See here for the phone number, etc:
http://london.usembas...sa/niv/interview.html
Chris