It really really depends.
Could be anywhere from $0 and 10 minutes of work to thousands of dollars and weeks of time to impossible.
If you know how to make a bootable USB for linux, you could doing that, and don't make any changes to the system, just see if you are able to mount your hard drive from inside linux.
If you know how to dismantle the laptop you could try removing the drive and putting it in an enclosure, and seeing if you could mount it on another computer.
If you aren't sure how to do either of those and your files are valuable, take it in to a shop. Maybe they can fix the whole computer for you.