I have experienced some issues in the past weeks with Options Bistro log in as well.
The first steps to follow are:
Close your browser.
Reopen it.
Go to wordpress.com
Log in.
Leave the window open and open a new browser window or tab
Go to OptionsBistro.com.
Refresh the page.
If that doesn’t work you’ll need to clear the browser cache. Follow these instructions.
https://en.support.wordpress.com/browser-issues/#clearing-your-browser-cache
Please reply in the comments here if any of the above resolves your issues.