Have you published your website to an online hosting service? Or to a folder on your hard drive?
The only way you can access your iWeb files is to have access to a Mac running Mojave or earlier. Then you can go into you Home/User/Library folder and look for the iWeb folder. Copy it in its entirety to the older Mac and place it in the same location as it was on your computer.
If you have published the iWeb site you can go to it online and copy and paste elements of it into a new web site page. The one that would work well in that situation is EverWeb . It has a similar GUI to iWeb and will let you start a new web page from a blank canvas. It will also let you copy and paste elements from a published website into the EverWeb page.
EverWeb has a demo version you can try. It will let you create webpages (but not for publishing) and review them in browser like window.
There are other web creation apps available at the App Store. Most of them are theme based which is OK if you don't have your own design you want to use.