Do the MMP files now open in Xmind or MindManager when you click on them? It's possible that Xmind has made itself the default program for MMP files. Assuming you are using Windows 10, go to Settings, select Apps and on the Apps menu click on Default apps - there are several options for changing file associations with programs.
In addition to the suggestion of Alex.
Mmp files were used before I believe vs 11 of MindManager.
The current file format is mmap.
So if you imported a xmind file and save it. It should be in mmap format.
Check if you have those stored in your default folder.