When you uninstall some programs their data files are left intact. For example, if you uninstall Firefox and then reinstall it again you'll find that your bookmarks and personal settings from the old installation magically appear in the new one.
So the data file which Rainlendar used
might still be present on your computer. Click on Start, type *.ics into the search box and hit Enter. That might find the file straight away. If not, in the empty search results box, go to Organize and click on the View tab. Click next to 'Show hidden files, folders and drives'. Click Apply and then OK and repeat your search.
[Note: Those instructions are based upon Windows 7 but they're probably similar in W10]
Hopefully that will find the file you want. If not it's probably gone for ever, although you could try Recuva to see if it can be restored:
https://www.piriform.com/recuva
If you can find the .ics file, move it to a new location (such as your desktop) so that there's no risk of a new installation of Rainlendar over-writing it. Then reinstall Rainlendar.
Once again search for *.ics, to find the empty data file which has just been added to your computer. Right-click on it and select 'Open file location'. Then delete the new .ics file from that folder and replace it with the old one.