Why could a single base mutation stop an enzyme from catalysing a reaction?

Because the single base mutation would change the primary structure of the enzyme which in turn would change the secondary structure of the enzyme due to the differences in hydrogen bonds formed. As a result the folding of the secondary structure to form the tertiary structure would also be different. Hence the shape of the tertiary structure would be different than it would have been without the single base mutation. As the tertiary structure of an enzyme forms its active site, the differently shaped active site would no longer be complementary to the substrate and hence the enzyme could no longer catalyse the reaction.