Reasons why you can not find what was online before is either contractual arrangements have expired or with the new rules having to follow European GDPR (General Data Protection Regulation)or worse yet, the web address for the site has changed.
Information of genealogical value is obtained from government agencies, religious groups, museums, genealogy societies, and other organizations under contractual agreements. The contracts specify what information is to provided, how it is to be made available and to what length of time it is to be shown. With the new guidelines in following general data protection, some sites or their data had to be removed. Companies have come and gone since the internet first appeared. Organization that posted the materials may have disappeared also.
If you kept the URL address you may have a source to check to see if the information is still available. The WaybackMachine may be your source to find the lost information. For more information on this website hit here.
If you find a record online that is valuable to you, save it now! Save it to your hard drive and make a backup copy someplace else as well. If there is no option to save, make a screen shot and save it on your hard drive and save another copy in the cloud or some other place where it will last for many years. Just because you can see the record online today does not mean that it will be available forever.