First I would do a rebuild with Ctrl+Q. If that doesn't fix it, open SolidWorks Explorer. Find the drawing and click on it, then click on the References tab. You should get a list of files that the drawing is referencing. RMB on them and choose Change. Change to the desired files.
Craig Cave wrote:
have replaced the original referenced assembly with a new assembly with the same file name.
This sounds like you changed references by moving the assembly that the drawing was referencing to another folder and replacing it with the new one. If that's the case, and neither of my suggestions above work, then try reversing this process.
Remember to close the drawing without saving it! It may be too late for this 'save' though. In any case you most likely have lost all dimensions, etc in that drawing as they won't be 'attached' to anything.