English question about usage of Nouns.

I have been looking for the usage of uncountable nouns for the last couple of hours. These nouns are really confusing me. Uncountable nouns like water, information normally cannot be used with articles. I was looking at the sentences below:
1. Food with a high content is bad for health.
2. I could not call you back since I was given a good work.

I think the nouns used here examples are countable as well as uncountable. Can the noun “work”: http://oald8.oxfordlearnersdictionaries.com/dictionary/work_2 be used the way it is used in #2 example? (see the example number 11 in dictionary.)
Similarly, “content”: http://oald8.oxfordlearnersdictionaries.com/dictionary/content is a singular noun, which means it should not be used with indefinite articles a/an.

We cannot calculate work, but dictionary says it can be used as a countable noun. Also, can anyone explain to me the difference between singular noun and uncountable noun or both are same?