Category permalinks not working properly

Hi.

I’ve been using a WordPress blog for over a year now, but I ran into a problem today when I clicked on a category archive link and found a 404 error page.

I tried to fix this in the permalink section of the options menu, and categories work fine when set to default permalinks (?cat=/?p=), but the links I used to use, which were pretty permalinks, became broken.

I have since changed back to pretty permalinks. When clicking a category, the link takes you to something like /2011/09/guides-2/, with ‘guides-2’ as the category. These pages don’t exist and bring up a 404 error.

While using this permalink structure, I can still access category archives by going /?cat=5, with ‘5’ representing the category number.

Is there any way to keep the ‘/2011/09/title/’ structure for post/page permalinks while also using the ‘/?cat=5’ structure for category permalinks by default?

I would massively appreciate any help.
You can see this (not) working here:
https://badasspanda.com

Thanks