HTTP Error 500

Hi!

I've just changed themes and started rebuilding my site, and then realised that I needed to copy a few codes in my old theme header. & footer. files into my new child theme.

However, as soon as I activated my old theme to access the header codes, my site crashed and came up with the following error..

The **** page isn’t working
**** is currently unable to handle this request.
HTTP ERROR 500

I'm nearly 100% sure that this is an issue with my old theme as this has happened before and thats one of the reasons I changed themes.

I have just requested FTP access from my server host, but only know VERY basic FTP. Once I'm in the FTP client, what do I do to fix this?!

Thank you so much in advance!

Mark