Good morning Sam and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the look of your calendar changing up on you.
I would love to help you with this topic.
The first thing I would recommend is heading over to Events -> Settings -> Display and trying to change the first setting (Default stylesheet used for events templates) by toggling it to a different setting and hitting “Save” to see if that helps.
If that does not work out, can you tell me if you have any type of caching plugin in place by any chance ?
Finally, if all else fails, you could trying downgrading to see if that fixes the issue: https://theeventscalendar.com/knowledgebase/downgrading-plugin-past-version/
Let me know how that goes.
Best regards,
Geoff B.