FIXING MyAlerts Dropdown Menu
This isn't going to be a fancy thread, as it's a straightforward one. There can be multiple ways to fix MyAlerts' dropdown modal not appearing, though most of the time it's just one easy fix. I was getting pretty frustrated with Alerts, and after tinkering around with my forum and settings for a little while, I found out it was one small change that needed to be made. Though that could be different for you.

These methods depend on the version of MyAlerts you're running.

1) This was my problem; if you use Cloudflare, and the Alerts menu stopped working after you enabled 'Universal SSL' or HTTPS on your site, you will need to go to your Admin CP > Configuration > Site Details and change any regular "http://" prefixes in the URLs to "https://"

2) The theme you're using may not support MyAlerts. You may need to manually edit this. You can check your templates by going to Admin CP > Templates & Styles > Templates > Your Theme Templates > MyAlerts Templates

3) Are you using the latest version of both MyBB & MyAlerts? You may need to edit /inc/plugins/myalerts.php. CTRL + F to find "compatibility", it should look like this:
'version'       => '2.0.4',
'compatibility' => '*',
'codename'      => 'mybbstuff_myalerts',
If not, correct that.
4) Ensure you have ClassLoader.php from Plugins Core in the /inc/plugins folder.
I can't think of any other issues right off the top of my head but MyAlerts can also have errors in SQL if you'd wanna check that too.
If you've upgraded from an old MyAlerts version, you should also ensure the myalerts_headerinclude file looks like the one found here too: