Saves are usually stored in C:/Users/<username>/AppData/Roaming/RenPy
AppData is a hidden folder by default. To see it, you need to enable "Hidden Items" under the View tab
If there is no RenPy folder under Roaming, it's possible the folder got deleted. I'd suggest check your Recycling Bin and see if you can restore it. If it isn't there, it's probably gone :(