installation impossible, grub not found.

[Arthur Schwing]

While trying to install Ubuntu in dual-boot with windows 7 pro on a Lenovo b51-80 installation could not be completed due to Grub could not be found. Big problem is that now the computer cannot even boot on windows 7. I get a blue screen due to hard drive corrupted while booting.
The hard drive was partitionned with a free partition reserved for Linux and formatted before installation. I don't understand why it corrupted the other partition.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14.12
ProcVersionSignature: Ubuntu 5.0.0-23.24~18.04.1-generic 5.0.15
Uname: Linux 5.0.0-23-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.7
Architecture: amd64
CasperVersion: 1.394
CurrentDesktop: ubuntu:GNOME
Date: Tue Nov 26 14:15:14 2019
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed cdrom-detect/try-usb=true persistent noprompt floppy.allowed_drive_mask=0 ignore_uuid boot=casper quiet splash ---
LiveMediaBuild: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
ProcEnviron:
 LANGUAGE=fr_FR.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 LC_NUMERIC=C.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)