I tried EssentialPIM Free version, liked it and purchased the Pro one. However, all my data was lost after installation of EPIM Pro. How to recover it?

EssentialPIM stores all data in the .epim database files. You only need to open (File -> Open) your old database in EPIM Pro. The default path for EPIM Free databases is: C:\Users\User_name\AppData\Roaming\EssentialPIM. If you can't find your file anyway, try to search for *.epim files on your PC.