I’m sorry you’re hitting difficulties.
In the first instance can you flush permalinks by visiting the Permalink Settings screen – you don’t need to actually change anything, just visiting this triggers a refresh of WordPress’s list of rewrite rules.
If that doesn’t help, please then try deactivating all other plugins and switch to a default, completely unmodified theme such as Twenty Thirteen (and then repeat the permalink flush step). Does that help here? If so, start reactivating everything one-at-a-time, testing at each stage to see if the problem returns and noting down what seems to be conflicting if so.
Let me know how you get on!