What happened to all the pictures on my facebook page ?

What you need to do is contact Facebook ( Facebook Help Page) and talk to someone there. They may let you take a picture of yourself with a piece of paper that says the date and some words, or they may make you answer questions about your account. This happens a lot, so I expect that they'll have some solution for you!

Facebook has a backup program on all its mobile variants, and since it's opt-out (and not opt-in), you need to disable the sync of your photos from your iPhone to your Facebook page manually. You can do this at either the system level or browser level:
System Level: disable Photo sync adapter under Facebook account sync in Settings
Browser level: Adjust the privacy of that folder so that only you can see its content.

You might try a different web browser like Firefox or Google Chrome, you can get them INSTALLED over at the link below good luck:

Ninite Install or Update Multiple Apps at Once

https://gethuman.com/phone-number/Facebook

You could try creating a new account and see if the problem keeps happening. The issue might not be with Facebook and could be with your browser. Contacting Facebook is like throwing a message in a bottle and hoping someone will find it and respond, customer service is not their forte

What Browser do you use to connect with ?

The only thing YOU could have done is to block her from your page and Facebook would have asked you if you were sure you wanted to block her.
This can be awkward, but if you are unable to get on your niece's profile, it is possible she deleted her account or blocked you. I would do a search in the search bar for her name and if she doesn't appear or you aren't able to get on her page you might ask her if she might have blocked you accidentally. Don't assume she has done something intentionally to hurt you, she may have made a mistake herself.

If want to deactivate your account select account settings option from the Account tab of the menu options at the top right hand side of the page.

Scroll to the bottom of the page and you will see the "Deactivate Account" click the deactviate link.

This doesn't mean you completely dissapear though!! "What this means is that you effectively disappear from the Facebook service. However, if you want to reactivate at some point, we do save your profile information (friends, photos, interests, etc.), and your account will look just the way it did when you deactivated if you decide to reactivate it."

If you would like your account permanently deleted with no option for recovery See below link for further details.
http://www.facebook.com/?ref=home#!/help/?faq=12271
and click the here link on the same page.

It is a problem with your account or with Facebook itself, neither can be fixed by you. All you can do it wait for it to be fixed or contact Facebook and open a support ticket to have it fixed.