Error 500 after changing web.config

I've managed to 'accidentally' take down one of my company's website by making a change to the web.config file. Short story:

I am in the process of designing a new master page, and during this I've tried uploading it to our Sitefinity environment to see if it works. I'm ran into a runtime error however, with the addition that I cannot see the error details because I'm not on a local machine (like this one). To remedy this, I have to switch the CustomErrors mode to Off, so I contacted our host provider to obtain a copy of the web.config. I got it, changed the proper line of code and sent it back with the request to replace the original web.config with this updated one. That was done, but the entire website, including the Sitefinity backend, now returns an Error 500. I can't reach or open any of it!

I immediately sent the original web.config file - of course I backed it up - the one with the CustomErrors mode not switched to off, with the request to put it back. I was told it was replaced back to the original web.config, but the site still returns Error 500. Basically one of my company's websites is now down while this error persists, so needless to say I'm desperately looking for a solution. 

This particular website uses the Sitefinity 3.2 system. Any idea what I could do to resolve the issue?