If you have accessed RE NXT in the past and this is a new issue, please do the following:

Clear the cookies and cache from your browser
Try to access from outside your network (use a mobile device and do not connect through office wifi, but use Data) or try to connect from home instead of office
If you are able to load the page from outside your network, but not from inside your network, please work with your IT team to allow connections to renxt.blackbaud.com