Anyone who has used WordPress for even a small amount of time will be only too aware of the pain when you try to access it and are confronted with the dreaded 500 Internal Server Error. It normally happens either at a busy time or when you want to publish a new post.
99% of the time the problem will lay with a problem with your .htaccess file. For reasons I am unable to explain the file can become corrupt at times. Some will say this is due to some plugins but I am not 100% certain of this.
Fixing the problem should be straight forward.
1. Back up your current .htaccess file. (This is incredibly important!)
2. Delete all of the information from your .htaccess file and save it i.e. upload a blank .htaccess file.
3. Your blog should now be usable again so go into the admin area and set your permalink structure again.
4. Open your new working .htaccess file in an editor. Now copy over all the non WordPress .htaccess information from your old backed up .htaccess file eg. any search engine htaccess information, ip blocks etc – basically everything Non-WordPress that you added before.
If you find that adding all the code from the previous .htaccess file is causing an Internal Server Error again then remove it all and enter the information back to the working .htaccess file line by line to see what is causing the problem.