I think it possible, because they have access to our laptop or open the hard drive in it. How to avoid?

1. Fix it yourself

2. Fix it in a trusted place 

3. View the repair process 

4. Encrypt your data and don't give password and username access 

5. Log out all the apps on your laptop 

Actually not all repairman are like that, but it is possible to do just that

Best solution, you can fix them by yourself

Possibly

Solution 1  you can make encrypted disk partition using veracrypt, virtual disk is entirely encrypted (including file names, allocation tables, free space, etc.) and behaves like a real disk. You can save (or copy, move, etc.) files to this virtual disk and they will be encrypted on the fly as they are being written 

Solution 2  you can use pgp make sure do not import your passphrase into device like password manager, save private key in other disk for example usb last encrypt your files 

Solution 3 fix your yourself haha