I ran a 2SLS regression in STATA, and there is no R squared value given in the output (only a dot where the value should be). What could this be interpreted as? Is it missing because it may be a negative value? And does this imply the output is not to be trusted?
The $R^2$ has no meaning with 2SLS/IV regression since the residual sum of squares is not restricted to be smaller than the total sum of squares. So to answer your question: Yes, it might be negative. Stata supresses printing a negative $R^2$ when you use the
For a detailed discussion of this question, see the detailed answer in Stata's FAQs which can be found here.