What happened?! I saved my file and now when I try to open it I get this message:

"The file being read is not a valid Poser file."

And then when I click "OK" it gives me a blank screen... What do I do?!

Which version of Poser?  And are you using External Binary Morphs?  If you're using P6 or P7, turn off the External Binary Morphs and then rebuild the file.  You might also be able to find a backup of the file but I don't remember what the extension is for it or where it's located.  Someone else will pipe up with that information, I'm sure.

Quote - I saved my file

 
Have you been tweaking the cr2 in a text editor? I get that message if I've been manually altering a character, prop or pose file, but missed a curley bracket out in the text, or inserted one in the wrong place (easy to do if you're cutting and pasting between two separate files). Fixing it relies on you remembering where you made alterations. Alternatively, you could grab a text comparison app from the web, and comparing the original file with the altered one. You did keep a copy of the original, right?

Quote - > Quote - I saved my file

 
Have you been tweaking the cr2 in a text editor? I get that message if I've been manually altering a character, prop or pose file, but missed a curley bracket out in the text, or inserted one in the wrong place (easy to do if you're cutting and pasting between two separate files). Fixing it relies on you remembering where you made alterations. Alternatively, you could grab a text comparison app from the web, and comparing the original file with the altered one. You did keep a copy of the original, right?

It was my original file... I never got the chance to save it twice. Its not too huge of a deal but yes I was switching textures on some props and that could have been what happened.

I have had this problem before !

My solution:

1.  open the uncompressed file ( CR2 not CRZ) in a text editor.  I use MetaPad - it allows larger file sizes to be read.
2.  check if a stupid idiot apostrophe mark or some other nonsensical punctuation mark has been inserted at the beginning of the file.  Delete that character mark !!
3.  re-save.
4.  check that it can now open in Poser.

Other symptom of silly characters being inserted:  when CR2s, PP2s and HR2s  are not even opened by Poser.

I should add - to uncompress a compressed Poser character, prop or hair, use the Compression-Uncompression Utility provided with Poser.  I think this could be in the Edit Preferences drop-down menu or under Python Utilities - you can it inside of Poser.