Hey there, tparamas.
Just a tip for future reference - you should never have the only copy of your data on a flash drive and you should always have it backed up somewhere as well.
Is the pen drive detected in Windows Explorer (My Computer)? If "Yes", do you get any errors when you try to open it? Is it recognized in Disk Management or Device Manager?
I'd recommend that you try the flash drive with a different computer/device, to see if the same thing happens. You could also try to access it via Ubuntu Live USB/DVD, to see if it's properly recognized and if you can get to your files. Try out some data recovery options as well.
Hope that helps. Please let me know how everything goes.