Are all language over $\Sigma= \{0\}$ decidable?
A language over $\{0\}$ is any subset of the set S of all strings consisting of a finite number of 0s. S is countable, the set of all subsets of S is uncountable. 

Any decidable language must be decided by some algorithm. The set of algorithms, no matter how you describe it, is countable, not uncountable. Therefore some subsets of S (actually almost all) are not decidable.