Internal server error - Drupal 6

I have Drupal 6 Site.

I have enabled compression of CSS & JS files. Last night I uploaded couple of css files. I cleared the Cache from Drupal site. It was working fine..

But this morning when I opened the site, it is throwing error

500 Internal Server Error. Contact your system administration.

I logged into as admin anyhow & disabled the compression so all the css & js files issue got fixed but the images that are under sites/default/files/ are not loading & getting 500 Error for these files.

This is very critical for my site.

I have already reverted the files & uploaded. But not working.

The strangest thing is that I was doing this on my subdomain dev.hello.com but the issue is also affecting / giving the same error on the main site as well. www.hello.com

Steps taken till now by me:

Reverted the old css & uploaded
Checked the files if they are available, they are available
.htacess intialised
https://www.drupal.org/node/1226512
https://www.drupal.org/node/587592
https://www.drupal.org/node/2620220

Exactly the same issue

https://www.drupal.org/node/702150

Any help highly appreciated.

I was in very critical need to fix. I google for almost 2 hours & tried lot things to fix it. But it didn't. Hence decided to post the question.

I have written very clear in my question,what links I followed, what could have caused the issue & PLUS highlighted that its very crucial.

But few people on this community are more interesting in down-voting,suggesting useless suggestion instead of finding the way to fix it.

Steps that fixed for me

Disabled the Optimization for a while
Navigated to sites/default/files, deleted the default .htaccess
Uploaded the new .htaccess
Cleared the Cache
ReEnabled the Optimization

Thats it. It fixed the issue

.htaccess

#SetHandler Drupal_Security_Do_Not_Remove_See_SA_2006_006
#Options None
#Options +FollowSymLinks