Do you have CS3 available? If so, try to open the file in CS3, then export to .inx and open that in CS5. It would also be worthwhile trying to get a new copy of the file. If it was sent to you via email or FTP it may have been damaged in transit. It is always a good idea to zip ID files before transfer.
The bad news is error code 5 is generally fatal, but I think John Hawinson may have described a way to look at the raw file and possibly correct the error using a binary editor -- not for the faint of heart. You may also have some success passing the file through Quark Xpress and back using the Markzware Q2ID and ID2Q plugins. Markzware offers a recovery attempt service if you don't have all the software required to do it yourself on a no-recovery/no-fee basis. See Fix Your Bad InDesign Files!
Err, only sort of. Peter is referring to this post: Re: InDesign CS5 "Cannot open [filename].indd Mac
where I decode some confusing instructions from a Markzware Youtube video.
I doubt they'll work in most cases, but it sure won't hurt to try.
I'LL TRY IT
THANKS A LOT,
I THINKK I WILL TRY MARZWARE.
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And all caps is considered shouting here. Upper and lower case is easier to read, too.