Scratch

I was working and saved and closed my project.  When i tried to reopen it it told me something about missing files.  so to solve the problem i took it to my computer-savvy freind.  but he said the file was perfectly fine.  this has happened to me twice.  once on a remix of the given "wodunnes world".  and once on a HALO parody called "super grunt".  The only thing in common that they have is the master cheif sprite that i tried to recolor yellow.  also on the remake of "super grunt" i got pretty far into it and the grunt won't go back to the standing costume but the costume is still there.  Could I maybe have too many variables?  how do i delete variables??  Please help me!!
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