I am troubled because I can not sign in to my e-mail. I am a "yahoo.com" user and there seems to be no entry point to accesses my account. There are many pop-ups indicating systems-for-pay that i encounter. Thank you for any assistnce you can provide. GMB
Either your page is hijacked or you have a virus on your system, do a scan on your local PC. Yahoo no longer supports certain browsers, but IE and Firefox should work. Delete all your browser cache and other windows cache and type in the address manually at the top (address bar not search bar).
Go into your browser's proxy settings and make sure it is set to no proxy (if your haven't filled it yourself). Do a Factory reset for browser. And as @Cyberat_88 said type in website name manually. there could be possibility that any proxy or plugin diverting your address bar path itself to any predefined location.