Internal 500 Error When Logging In

I’m working on a client’s website. It loads fine, but when I try to log in to the backend, I’m getting this error.

Error 500 – Internal Server Error

An error was encountered while processing your request. Typically this is a temporary condition. Please contact the web site owner for further assistance.

I searched the internet for this type of error. One possible solution was to rename the .htaccess file. I did that, but I’m still not able to log in. I don’t want to rename the plugins folder yet because the website is still working, and I don’t want my client to lose any business. One of the plugins installed is Gravity Forms, and the website is using the Thesis theme.

Can anybody help?