I am using the img resizer on a bunch of sites and I had a server upgrade and since then all images will not resize. I think it is connected to a php plugin but am not sure which one I need to enable on my server
Do you have the latest version which is 2.6.4, if not upgrade and see if that solves your issue.
Are you aware that this plugin is no longer in development and that it is effectively abandonware? For a more robust solution that is actively developed and supported by the developer I would recommend CE Image additionally the documentation is second to none making it very easy to use.
Most likely some issue with the GD library. Check for errors for further hints: Image Resizer doesn't output errors but logs them in the Template Parsing Log. You can turn that on in the CP at Admin → System Administration → Output and Debugging and switch Display Template Debugging? to yes.