Can I change my field for Masters? I just got done with my bachelors in Computer Science. After four years of CS, I realized I don't really like programming or anything to do with computers. I'd rather work for organisations which solve real world problems. Anyhow, I was wondering if I could get admission in Economics or international relations or something. Will I be able to get into a good university (I'm interested in European unis) and what future prospects would I have as a professional?
That depends on the requirements / prerequisites of the specific program you are applying to, but Masters classes assume you already understand the topic at a Bachelors level and unless you have at least a strong minor in that area you are unlikely to succeeded and thus unlikely to be admitted.

A better approach might be to ask schools if they would let you use any part of you existing degree as "transfer credits" toward a faster Bachelors degree,  then continue from there to the Masters.  You might be able to complete the new B in three years, or even two, if you have good enough grades in material that overlaps their program.