I am a US citizen who had more than $5000 USD stored in MtGox at the time of its collapse.
What action can I take right now to work towards getting my money back?
Citizenship is not important. You should file a claim with the Japanese court-appointed liquidator, Mr. Nobuaky Kobayashi. Links and information were posted in this thread a couple of weeks ago; look for "creditors meeting". Some information is available in the MtGOX original website
http://www.mtgox.com, but that site may still be managed by Mark, I dont know.
The deadline for filing claims was Nov/2014 but has been postponed to May/2015. There is no date yet for when the spoils will be distributed. Be prepared to get only 1/5 of your claim, at most.