Hi ed,
I’m sorry you’re having trouble with 404s. This can happen after another plugin (or sometimes even a theme) adds its own custom rewrite rules, effectively confusing WordPress and stopping it from finding resources like events pages.
The quickest possible path to a solution is simply to flush your rewrite rules: this is as easy as visiting the Permalinks Settings admin screen: you need change nothing, just visiting this screen makes WordPress clear up its list of rules.
If that doesn’t work by itself then the best course of action is to follow our standard troubleshooting steps: start by ensuring your events plugins are up-to-date. Next, deactivate all other plugins and, switch to a default and unmodified theme – such as Twenty Thirteen. Finally, repeat the above step for flushing the rewrite rules – then check again and see if your events pages are accessible.
Best,
Leah
