Sometimes you may reinstall you Windows OS keeping your Ubuntu partitions. But after rebooting you can see that the grub is replaced with the windows boot loader and you no longer will be able to boot to your old Ubuntu OS.
But you can simple fix this issue by booting with a Ubuntu Live CD or USB. Here is the step
1. Boot your system with a Ubuntu CD or a Ubuntu live USB.
2. When it boots choose “live CD” option and you will be presented with a Ubunt Desktop.
3. Connect your system to Internet via Ethernet or Wifi.
4. Now open a terminal and run the following command. sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update sudo apt-get install -y boot-repair && boot-repair
5. After installing, “Boot-Repair” program will come up. Now continue with the prompt and Choose the first option which is “Recommended Repair”
6. After you get a successful message, Reboot your PC and remove the Live CD / USB.
Your Grub boot loader is back. Now you can use your old ubuntu and windows 
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