It’s normally one of these things:
1. Your subscription isn’t active. This is normally due to a failed payment for the subscription. It happens! The website will automatically suspend your access if your sub isn’t active. It can be easily rectified by making the missed payment. You can check your account status here.
2. If your account all looks ok then it’s likely the website has signed you out. This can happen occasionally due to server or software updates. This can be sorted out by deleting the podcast from your chosen podcast app and then adding it again, as per the instructions on this page.
If those steps don’t fix it then get in touch and we’ll look into it.