The first thing to do is edit all of those posts.

Change the content and title to anything you like. Find a funny animated gif or something.

How come the hacker got my account?

This is the most likely explanation of how it was stolen: STEEMIT PHISHING SCAM WARNING!

Now that my level is 5, how will this affect me on Steemit?

After you edit all the posts you can mention me in a comment or post, or find me on steemit.chat and I'll remove the downvotes from @steemcleaners & @patrice.

Is there any way to protect myself from this kind of attacks in the future?

Learn about what your keys are for and login with your posting key unless you have to transfer money. Then login with your active key. Secure your owner key or password by keeping in a safe place. I recommend keeping it in several safe places.

Dummies Guide to Basic Steemit Account Security + Account Recovery Guide! Must Read For Steemit Users!

Is there any way to protect myself from this kind of attacks in the future?

Be careful of clicking links in posts. If the link takes you to another site that looks like steemit.com check the url at the top of the browser.

!cheetah unban

I have removed you from the blacklist. As soon as you edit those posts I can help you recover your reputation. Or if you wish to get a new account I can help with that too. I'm @patrice in steemit.chat and on Discord