Can't access dashboard

All of a sudden when I go to my website /wp-admin I can see this above the log on and then I see this:

“Warning: Cannot modify header information – headers already sent by (output started at /home/andyscar/public_html/wp-content/themes/customizr/inc/parts/class-header-nav_walker.php:74) in /home/andyscar/public_html/wp-includes/pluggable.php on line 900”

then I can sign in below and I receive this:
ERROR: Cookies are blocked due to unexpected output. For help, please see this documentation or try
the support forums.

Sometimes when I try I won’t even get a log in I will on I will only see this information:
Unauthorized
This server could not verify that you are authorized to access the document requested. Either you supplied the wrong credentials (e.g., bad password), or your browser doesn’t understand how to supply the credentials required.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

I have tried logging on from different computers at different locations to see if maybe it was something from my computer and I am still having the issue. I contacted my host site and they said to contact you all. I dont know what to do, I am just trying to work on my website and I can’t.

Thank you for your help!