Hi Steven,
Sorry to hear about this.
Have you tried re-downloading the Event Tickets Plus plugin from your account and install that?
If you hit an error, please create a screenshot and share it with me.
It is possible (although unlikely with that error message) is that something went wrong with the previous installation. If you still have an old version of ET+ on your site, then you can go for the manual update: deactivate then delete the plugin. Download the latest version from your account, upload it under Plugins > Add new > Upload then activate it.
If you don’t see the plugin in the list then you can confirm it’s non-presence via logging in through ftp and checking if the wp-content/plugins/event-tickets-plus folder exists. If it does, then delete it.
Let me know what gives.
Cheers,
Andras