I have recently enjoyed playing divinity original sin and when i play the game a few weeks later alot of save files are gone.
I also started to play nwn enhanced edition and played Undertide. The save files cant be found?Is this common? What happened to the save files?

No, that should never happen. Are you using galaxy and cloud saves?
Also. make sure you never install to Program Files or x86.
Other than that your files should remain present, so it sounds like something up with your machine. Have you run a full virus scan/malware scan. Make sure your not installing to removable media etc.

In case you didn't know the save game locations (couldn't tell from what you wrote) - https://www.pcgamingwiki.com comes in handy in this regard:

pcgamingwiki.com/wiki/Divinity:_Original_Sin#Save_game_data_location
pcgamingwiki.com/wiki/Neverwinter_Nights:_Enhanced_Edition#Save_game_data_location