Malicious executable code

Can anyone help me with this? I am a complete technical novice!

I ran a Wordfence scan and recieved the following message…..

This file may contain malicious executable code: /home1/l1l2a3/public_html/wp-content/themes/flexsqueeze150/404.php
Filename: wp-content/themes/flexsqueeze150/404.php
File type: Not a core, theme or plugin file.
Issue first detected: 30 mins ago.
Severity: Critical
Status New
This file is a PHP executable file and contains an eval() function and base64() decoding function on the same line. This is a common technique used by hackers to hide and execute code. If you know about this file you can choose to ignore it to exclude it from future scans.

I now seem to have no access to my wordpress dashboard (page saying: no data received) and Avast warns me of the following:

URL: https://blazingsables.co.uk/wp-admin/index.php|{gzip}
Infection: JS:ScriptIP-inf[Trj]
Process: C:\Program Files (x86)\Google\Application\Chrome.exe

Can someone please tell me what this coding might look like and how to remove it please? Flexsqueeze is the theme I am using for my website and I don’t want to delete the infected files and lose everything that I have done!

Thank you :/