Can someone help me fix a "greyed out" save button for a new contribution?
This is likely to be a resource URL issue - the way these fields are calculated has changed in CiviCRM 4.7.

Please look at Administer menu » System Settings » Resource URLs and Administer menu » System Settings » Directories.  Please ensure that both of these look correct.  Note that you might see a token (e.g. [cms.root]) as part of your URLs and paths - you can press the little question mark in the blue circle at the top of the page to see what those tokens expand to.

[See comments below for my final fix which was to to go to system settings -> clean up caches and update paths - JR]