It´s a UEFI. But Lenovo is still calling it a BIOS. It suggested me to actualize it. Now I think it went back to it´s default settings and boots Windows automatically without asking.
I guess that the efi entry for Ubuntu has been deleted.
The only way to fix that is to use the boot repair method described here
I don´t have a CD reader. Can I Use the USB flash drive I used to install Budgie as the Ubuntu CD?
Yes. Just write the iso image to a usb stick and boot into the live session.
Sorry. It worked and I forgot to report. Thanks a lot.