Library cleared and completely reset after update to 5.5

Same thing happened to me.

Did you update via rekordbox or did you manually download and install?

rekordbox itself doesn't overwrite the settings files or database(s).

What OS (version / build) are you running?

Upon launch I was offered an update. Ran it from there.

Can you open this folder and tell me if there are any datafile.edb or datafile.backup.edb files in there:

C:\Users\yourusername\AppData\Roaming\Pioneer\rekordbox

I have to say I don't know what it is but it took me 10 min to get a browser to work with this forum board. Pioneer needs to get a better message board. When I go to log in it brings me to a profile edit page and I'm already logged in. I try to click products to get to the forum topics and it brings me back to my profile login page. 

Some how I finally got to this thread where i can reply. 

Yes  I have three  datafile.backup.edb files labeled 

Two are dated for 4/27 one is dated for 4/28.

Windows 10 build 1803

The same thing has happened to me now twice. Both times after applying windows 10 updates. It's beginning to get annoying. 

The only way rekordbox would revert to a blank database is if there was some damage / corruption to the settings file so it wiped out the old and created a new database. As this is extremely rare, I'd suggest you please file an inquiry ticket here and our support team will gather some additional information to assist you in resolving this issue.

I don't think it's a rekordbox issue. My mixed in key has gone also, found the installation files and all the settings were blatted, the same as Rekordbox 

https://answers.microsoft.com/en-us/windows/forum/all/windows-10-updates-removing-apps-from-the-taskbar/a4eb665e-c3ea-4a93-9c35-d599f14b0779