First, my apologies if the question has already been answered but I looked and didn't find anything that completely answered me. And thanks to those who'll answer.
My problem is not only about starting Windows after Ubuntu was installed: they were installed on separate disks.
First off, I use a Dell Vostro. I got it with Windows 10 and a 250 Go SSD and thought I'd add some space, so I added a 1T SSD on which I wanted to install Lubuntu. The new SSD was recognized by Windows, no problem. Then I made my Lubuntu USB, tried Lubuntu which ran great. And I tried installing, four times.
The installer didn't see Windows and only proposed to erase everything. When I asked for "something else", my original SSD didn't exist there. Then I read about how to partition a disk on the installer and followed these instructions : https://help.ubuntu.com/community/DiskSpace.
Now Lubuntu was installed fine (on the 1T SSD) and runs without trouble, but I can't access Windows unless I disconnect my 1T SSD (I tried it, Windows is still there and runs when only the 250G SSD is on). I tried hitting F12 while the computer boots and using Windows Boot Manager instead of Lubuntu, but it acts as if Windows was missing entirely.
What can I do to make this work? Not afraid of reinstalling Lubuntu some other way if needed.