Adobe Flash is being discontinued as a product in 2020. Flash player has long been criticised as a vector for security vulnerabilities and the slow performance of webpages.
See Adobes own announcement: https://theblog.adobe.com/adobe-flash-update/
Major browser applications (Google Chrome is a web browser developed by Google and is available on most school PC's. If you are having trouble with certain websites, it is useful to try a different web browser to see if this fixes the problem., Microsoft Edge, Firefox) have already started the process of phasing out the ability of websites to play flash content, this will most likely be the cause of you not being able to play a favourite resource on a website you have been previously been using for years. And programs such as Microsoft Office will drop support for flash content in presentations.
What does this mean for me as a teacher?
Overall the onus is on website owners to update their websites to use more modern technology under the umbrella term “HTML5”.
In the short term you can still run flash content using Internet Explorer, as it still supports Flash on all websites that require it.
In the long term it is advisable to look for replacement resources if it is apparent that a website is unlikely to update its content. The term “HTML5” is very useful when searching for modern sites. For example, if you were looking for a website that shows chemical compounds, you could google “Chemical Compounds HTML5” and you would have a fair chance of bringing up websites that are fully supported by modern browsers i.e. molview.org