Macbook pro displaying only vertical gray lines after Apple startup logo

I'm sorry, but it is hard to say. Usually if the external display appears Ok, then the issue is limited to the laptop's built-in Display, however, these 2018+ laptops are very difficult to troubleshoot as they don't behave in the traditional manner all the time.

Make sure to have a good backup before taking it in for repair.

You should always have frequent and regular backups. It is impossible to recover accidentally deleted data from an SSD after the Trash has been emptied plus an SSD can fail at any time without any warning signs. In addition any 2018+ Macs with the T2 security chip or M1 CPU has even more ways to lose your data if any of the hardware has an issue that keeps the laptop from booting. Anything that corrupts the T2 security chip such as a simple OS update that goes wrong can cause you to lose permanently lose access to the data on the SSD unless you have a backup. Even an expensive professional data recovery service will not be able to help you since the data on the SSDs are hardware encrypted which requires a working T2 security chip and working Logic Board.

FYI, iCloud is not a backup option for your computer.

Good luck.