For some reason, the census API seems to be down today (Sat March 26, 2016), so you may be getting errors when you try to fetch data. They look like this:
Error in file(file, "rt") : cannot open the connection In addition: Warning message: No data found at: http://api.census.gov/data/2014/acs5?... [with lots more stuff here]
The error has nothing to do with the acs package or R, but is a census-side problem. To my knowledge, the API has been pretty reliable, so let’s hope it’s back up soon. Sorry for any problems all the same.