Use Secure URLsin Admin Configuration
Yesthen my full admin panel has been changed.
See if your problem is getting fixed by clearing the Cache (System -> Cache Management). You may need to do this manually by emptying the contents of var/cache.
If you are using CDN like Cloudflare, make sure all files are purged.
If you are sure that the URL's are correct and you have SSL correctly set up then clearing your cache should fix this issue, as suggested by Renish K.R.
Since you can't clear the cache from Admin you will have to do this manually by deleting the contents of var/cache in the Magento root directory.
If you don't have SSL correctly set up, you will need to manually revert your change by editing the relevant values in core_config_data which is covered here: How do I fix my Base URLs so I can access my Magento site?
You can revert your changes by going into the db, table
core_config_data and remoging the rows with
clear the cache when you are done.