There could be many reasons why it is not booting up: Broken packages, some issue with kernel, maybe you stopped some installation in between, forced shutdown with power button or in worst case could be hardware failure. But it could be fixed.
Fixing Ubuntu:
-> You might see an option "Advance option for Ubuntu" in the GRUB menu. You can get the grub menu by pressing Shift or Esc key. There you can go th recovery mode and bunch of options are available to repair the system, try that.
-> If above does not work then you have to make a bootable drive and can reinstall the ubuntu. If you want to recover you data then go to 'try ubuntu' option and mount your /home directory from other devices in file explorer or using terminal.
Refer this:
https://www.howtogeek.com/196740/how-to-fix-an-ubuntu-system-when-it-wont-boot/
Recovering Files from ubuntu via Windows:
Since you have done dual boot with Windows. Go to boot menu (check boot key for your laptop/pc on google). Select Windows. If it successfully boots Windows then install Diskinternals. Using this you can access all your files from ubuntu.
If your Windows is also not booting up then maybe while installing ubuntu you may have installed it along side windows in the same disk partition. Due to which corruption in ubuntu caused corruption in Windows. In this case first try to recover your files from above methods and then recover or reinstall Windows with windows recovery mode.
Some Suggestions:
I would recommend installing ubuntu and windows on different disk partitions as issue in one can lead to issue in another. Also it is safe and easy to access all the files even if one gets corrupt.
Also, it is better to make /root and /home partitions separately while installing ubuntu. Because even if your ubuntu gets corrupted you could easily remove and reinstall /root partition with all you personal files safe in /home directory.
