Facebook account can be hacked if -
Hacking of Facebook Account:
If you think your account has been hacked, you should change the password and review the recent login activity.
Your Account may have been Hacked if you Notice:
Important Note:
If the email associated with your Facebook account has been changed, you can reverse this but within limited time-frame. When an email changes, Facebook sends a message to the previous email account with a special link. You can click on that link to reverse the email address change and secure your account.
Before registering complaint, find whether you are able to access the account?
Have Access:
Use various methods to strengthen the account. This includes –
1) Change Password: Settings >Security and Login >Change password >Save changes
2) Change Number: Settings >Personal Information >Phone Numbers/ Edit and Add number
3) Logout from all the Devices: Settings >Security and Login >Where you’re logged in >Not you?/ Logout
4) Download the Information: Settings >Your Facebook information >Download Your Information >Create file
To check the alterations/ activity done by the hacker
Access Denied Case:
If the hacker has changed the password and mobile number, there are 2 possibilities.
Quick action is required in cases where hacking can be “Suicidal, Defamation, Threat to life, Threat to national peace, Demand of ransom, Cyber Terrorism etc”.
Precautions:
Prefer the extra security settings for the account.
Settings >Security and Login >Setting up extra security
Tap on “Edit” next to “Get alerts about unrecognized logins”. This let you know if anyone logs in from a device or browser you don’t usually use. Thus, provides alerts when anyone logs in to your account from an unrecognized device or browser.
You should also turn on “Two Factor Authentication” for extra security. 
Review “Authorised Logins” under Two Factor Authentication settings.
Settings >Security and login >Authorized login
You should review the devices on which you won’t have to use a login code.
Relevant Documents:
You should provide the following details to police, for proper investigation.
1) Facebook User Number:
A number used to identify accounts uniquely. The process to extract it –
Example: 
The above page shows the unique identifier for Mark Zukerberg - “4”. Normally, users have much larger user number like “1000000000000000000000004724823323”.
Or follow the following steps:
You will find the User Number of the target profile
How to Report Hacking on Facebook?
Step 1: Navigate to - https://www.facebook.com/help/
Step 2: Tap on “Policies and Reporting” menu and Select “Hacked and Fake Accounts” from the drop down menu.
Tap on “I think my account was hacked or someone is using it without my permission”. Then tap on “visit this page”. It should open like – 
Select the appropriate option and follow on screen instructions. For example, you may select “Someone else got into my account without my permission” as per your case, and then tap on “Continue”. 
Tap on “Get Started” and follow the on screen instructions.
