Restrictions can be set to Limit Adult Content. Settings > General > Restrictions. It's passcode protected. Don't forget the passcode if you set it.
That's the only thing I know of that will restrict website content.
If that is not the issue, try going to Settings > Safari and Clear History and Website Data. Some sites need to store data on your iPad to work, and if it gets corrupted it can cause problems. Clearing this out will force the site to download the data again.
Also, make sure you are not in Private browser mode. The title/tool bar will be a dark gray instead of white. In this mode the iPad can not download or store the data/cookies that some sites need to work properly.
You can enter/exit Private browsing mode by tapping on the double square icon at the top right of the tool bar then tapping on the Private button.
The last possibility is that the site uses Flash or Java. Safari does not support these web technologies. There are, however, some third party browser that do, in a round about way. They basically give you a remote session on a server that does support Flash and/or Java, and show the site on the iPad.
One of the best of these, in my opinion, is Puffin. There is a free trial version you can try out for a few days.