I just changed my theme and want to revert back to the one I had before as it has deleted all my changes. I have already reverted to the original theme but my changes have not come back, how do I get my work back?
I'm afraid you will need to get a backup for your previous theme. But the question is whether you ever made it.
If you are lucky you don't have to overwrite all web pages again with update and / or publish command. Then I would be the first to see what can still be found online from your old website.
Then you can request this page by page and save it as a file off-line on your PC. In the worst case, you can rebuild your old web pages again and use the stored text again
You could also ask if the helpdesk can help you by creating a ticket request for a restore.