Catalina Upgrade - Purchased Apps Gone

You say that you "lost" all of your purchased apps. I don't know what "lost" means. Are you saying that you are now unable to open them? If so, then your problem likely is that those apps are 32 bit apps and Catalina now only works with 64 bit apps. You didn't say which apps you "lost". Some may be able to be upgraded. I suggest checking the support web pages and see which apps have upgrades which can be downloaded. If Office is one of your problems, be advised that you can only upgrade Office 2016 or later. If your version is older, then you will have to pay for a newer version or convert to using an alternative such as LobreOffice or OpenOffice. Or you could downgrade to your former OS. The Apple help site tells how to do this. If this is not your problem, please tell us specifically what the problem is.

Did the same when I downloaded. Will not recognise photoshop 5 or Office programmes. Tried to get rid of it and get back to High Sierra but ended up erasing everything. Now in shop costing me big bucks

I am sorry you had to send it to the shop. Just so you will know for next time, the Apple help site has complete and specific instructions on how do downgrade an operating system yourself. So, next time before you spend money, you can check there (or here) and receive some help.

Final comment. At some point you will likely have to upgrade your operating system for any number of reasons. When that time comes, you will again be faced with the need to upgrade your old 32 bit apps. Also, the app vendors will not continue to support older versions indefinitely, which can cause future problems. Long story short, is that at some point all apps need to be upgraded for a variety of reasons. Some can be upgraded at no cost and others will have an upgrade cost, just so you know. Good luck.