I was working on my arch linux for like two months and everything was fine until i had to install windows 10 to use some softwares, after installing windows 10 i couldn't enter my arch! there was no dual os menu at the starting time so right now i'm using windows 10 ! i can't just uninstall it because i installed it on an important partition i can't just format it to erase the windows , so right now i've got like 3 partitions : arch 100Gb ,swap 6Gb and windows + all my files 850+ Gb .. could someone help me ?!
Windows setup is known to overwrite your MBR. In other words your GRUB2 bootloader is gone, so you need to restore it.
You can use Super Grub2 Disk to do this easily.