"A few cash" is incorrect. You can only use the word "few" with things that can be counted, and "cash" is not a countable word. You could, for example, say "A few dollars," because dollars can be counted, but with "cash" you must use the word "some." Also, using the passive voice you shouldn't say "is stolen." The proper way to say this is "was stolen." So, a better way to say this sentence is "Some of my cash was stolen."
Not if you mean that some of the cash you have is stolen cash...