This is not a serious problem, all I have to do is re-download, and usually if you have already downloaded a module it reloads in seconds. That tells me it’s still downloaded, but not in the list. I don’t know, but I’m guessing.
The problem is I have a lot of modules, usually the same ones that disappear from my library every time I close and open the Vassal library. Does anyone know why, and if there’s anything I can do about it? 
Modules disappearing from your list in the VASSAL Module Manager view is not an indication that they need to be re-downloaded. The module files themselves–wherever you keep them on your hard drive–are still present. Doing File → Open module and navigating to where the module is saved will restore it to the module list. This doesn’t solve the issue of why they’re disappearing in the first place, but just be aware. Where do you have VASSAL installed, and in what location on your disk are you storing modules?
The app and the files I have made for current games are in documents. I think you about to advise me that I can just open the particular game and that will restore the module to my vassal library and open the app as well. I have discovered this, but none the less, appreciate you being helpful
I am having trouble also. All of my modules disappeared. (HELP! )
Here is where I can a listing (example):
C:\Users\Dave\AppData\Roaming\VASSAL\prefs
But when I try to open anything it always does nothing.
The only thing I can think of is that windows 10 had some thing keeping the computer from shutting down and I found a way to suppress it but dont know what that is any longer since that was months ago.
Does anyone have any suggestions?
Thank you!
-Dave
It has just happened to me, even though I have not updated Vassal, all I did is reboot my PC.
If modules disappear from the Module Manger, it would help us to see the errorLog from that run. There’s not much we can do without that.
Reposted from other thread:
So after it happening all the time not just upgrades (maybe after upgrades has stopped) for years I can now confirm: as I store my games (the actual modules) on Google Drive to prevent me having a copy on every device/having to download it on every desktop and laptop I use. If I ever open Vassal (the engine is stored /installed locally) within 30 ish seconds of booting up (before Google Drive is ready) OR if I ever load Vassal when Google Drive has crashed, Vassal will presumably try and find the modules and when it fails it just purges them from the list. Now I’m sure there’s some logic there, but in the same way if someone stored their modules on a USB pen: just because the path may not be valid at any given time, doesn’t mean it will never be valid again. My solution would be don’t perform the check at all (check only when the module is requested to load then generate an exception/error if needed) or if the check must be performed just change the colour of the text identifying that module to red or something rather than deleting it from the list?