This is a common problem and there are a number of possible solutions, addressed below.
The most likely solution: clear you browser cache/cookies/site data and then search again. This sometimes happens when a browser session times out or a cookie has otherwise gone 'stale.'
Search again in an incognito browser window. An incognito window will not utilize saved cookies and will afford you a 'fresh start' to enter your SJU credentials.
If you recently changed your password and your browser has your old password saved, you should clear your cache/ browsing history, then log in to SJU with your new password.
It’s uncommon, but sometimes individuals have a script/cookie/popup blocker enabled on their browsers. If you have any of these plugins or means of blocking enabled, disable them temporarily, or try an alternate browser that doesn’t have them installed.
Additionally, there are some commercial anti-virus products that also “protect” a browser by enforcing the same “no-script/no-cookies” paradigm. Ultimately, if the browser is unable to store a cookie, SJU's proxy server will see that there’s no cookie, and will repeatedly re-prompt for authentication.
Lastly, if you have a home VPN setup, or some high-security router, it’s possible that the proxying of the IP address is being blocked or otherwise prevented. If possible, attempt to access library resources over your smartphone, or even another computer if available. If you can access databases via these other means, the problem is isolated to your browser or computer.