I loaded the project on a different computer than I usually work on and all block definitions said undefined and I accidentally saved it and now the computer that it worked on before doesn't work either. Is there anyway to retrieve a previous save?
Yes. First of all, on your usual computer, from the File menu 
choose Open, type in your project name, then instead of the Open button push the Recover button. Find a version that predates your save from the other computer.
That should solve your immediate problem. Here are some tips for later:
First, when something happens that you don't expect, do not save the project! If you must, do File > Save as... and then give it a different name.
Second, the reason that other computer gave you all those red Undefined blocks (and yeah, this isn't obvious) is that it has an obsolete version of Snap! in its browser cache. Do a hard reload (shift-command-R or shift-control-R probably) to fix it.
Are you a student in a Snap!-using class? Which curriculum? I ask because your teacher ought to have warned you about this; we warned the ones we know about.
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