Hi,
Method 1:
Resolve warnings. To ensure that the Not Secure warning is not displayed for your pages, you must ensure that all forms containing elements and any inputs detected as credit card fields are present only on secure origins. This means that the top-level page must be HTTPS and, if the input is in an iframe.
Refer to the link provided below and check if it helps:
Change Internet Explorer Security settings
Method 2:
Clear the SSL state
In some instances there may be corrupted information in the Secure Sockets Layer (SSL) cache; this corrupted information can cause websites to not load correctly. This information is cleared each time you restart your computer but can be removed by using the Clear SSL State option in Internet Explorer without having to restart your computer. To do this, follow these steps:
a)Click Start, type Internet Explorer in the search box, and then, in the list of results, click Internet Explorer.
b)In Internet Explorer, click Tools, and then click Internet Options.
c)Click the Content tab, and then click Clear SSL state.
For more information refer to the Kb article:
Cannot access some websites in Internet Explorer
Also refer to:
Security zones: adding or removing websites
Hope this helps