This is probably not fast solution to the issue, though it worked for me

What happened

I had several operating system partitions (dual boot) on one HDD, first one [sda1] was Windows 7, second [sda4] Linux.

Both had common grub boot menu created by Linux upon its install (as a secondary OS).

Then i reinstalled Windows, formating its sda1 partition during its install. Result was that Windows overwritten boot loader and i can not select to boot into Linux.

What i did and what worked

I booted my MX Linux install flash drive [created in Rufus [app] from .iso image], live system booted, then i found "MX Tools" in MX menu. One tool was MX boot repair app. inside i selected to backup MBR and PBR to my external flash drive and then tried "Repair grub configuration file" option which resulted in:

Generating grub configuration file ...

using custom appearance settingsFound Windows 7 on /dev/sda1

Found background image: /usr/share/backgrounds/MXLinux/grub/MX-Gold1080P.png
Found linux image: /boot/vmlinuz-4.15.0-1-amd64

Found initrd image: /boot/initrd.img-4.15.0-1-amd64
Found Windows 7 on /dev/sda1

Found linux image: /boot/vmlinuz-4.15.0-1-amd64

Found initrd image: /boot/initrd.img-4.15.0-1-amd64

Found memtest86+ image: /boot/memtest86+.bin
Found memtest86+ multiboot image: /boot/memtest86+_multiboot.bin
Found memdisk: /boot/memdisk

Imagepath /boot/images not found
Interesting line:

Found Windows 7 on /dev/sda1
then i restart PC, but still only Windows, no menu to choose OS at boot. So i booted the MX Linux live again from USB drive and in same tool like before ("Boot repair") i selected "Reinstall GRUB loader", in next step selected MBR [i do not have ESP/UEFI Bios], location was sda, /boot location was MX linux partition sda4.

After reboot i was able to see the boot menu to select between Windows and Linux.