Hi Nicola,
When there is a 500 error on a website, something will be logged in the webserver’s error logs. So, the thing to do is find the IIS and PHP error logs, and see what it says in them.
Possible issues are that perhaps your new webserver is running a significantly different PHP version from your old one, and you may have plugins or themes that aren’t tested/supported on your new PHP version. Or, if you’re now on a Windows server and were previously on something else, the plugin/theme authors may not have tested on Windows. But, this is just a first guess. If you find the logs, and see what’s there, that’ll give something substantial to proceed with.
David