I am dual booting windows 10 and Ubuntu 18.4 and it worked fine till now. Today I encrypted the Linux partition of my hdd using Veracrypt and since then the Ubuntu always boots into gnu grub version 2.02. It seems that I can not get out of there. Anyone knowing how to solve this problem? Many other people face problems with grub booting but I haven't found any similar incidence that has to do with disk encryption in any forum.
When it comes to encrypting system volumes, Veracrypt can only encrypt Windows systems running in BIOS boot mode. Documentation link
Unfortunately since the files Grub uses to boot your Ubuntu and Windows operating systems are stored in the now encrypted Ubuntu Partition, you will have to perform steps to restore the Windows boot record as described here: Windows boot recovery instructions. Once this is done you can use Veracrypt from Windows to decrypt your Ubuntu partition.
After decryption, next you will have to reinstall the Grub bootloader so that you can boot to Ubuntu again. The simplest way to do this is to boot into an Ubuntu installation media and then follow the steps here to repair and reinstall grub to your system's boot record.
To the extent of my knowledge there currently is no way to encrypt in place an existing Ubuntu installation and still have it be functional as an operating system. If you are willing to reinstall your Ubuntu installation you can attempt the steps outlined in the full disk encryption guide, but there is no mention in it of supporting dual boot.