HTTP 500 error with NextGen

Please help! I am an admin on a website that does not belong to me…it belongs to my boss. I installed the NextGen plugin to do some photo galleries for her and was immediately locked out with the HTTP 500 error. Since I can’t get past that error page to log into my admin account, I can’t uninstall NextGen, which I’m seeing is the only way to fix this. How can I bypass the error page and get back into my account so I can uninstall this and keep working? I am on hold til I figure this out and my boss is not happy. TIA
Edited to add: She does not have the current version of WordPress…one of my tasks this weekend is to upgrade it