Don't trust any of the many advertised online PDF repair utilities. I've tested a whole bunch of them, and none of them was able to actually fix a corrupt PDF that would not also be fixed (silently) by Adobe Acrobat or the free Adobe Reader.
If you only used "File>Save" on the file, and not "File>Save As", there may be a slight chance that you can restore a previous version of the file. You need to open the file in a binary editor - not a normal text editor, and definitely not in a word processor. It needs to be something that will not change and re-interpret data for you while you edit the file. I've used NotePad++ on Windows, and TextMate and vi on a Mac for that. After you open the file, you have to be very careful to not accidentally change anything that you do not want to change.
Before you do anything, make a copy of your file, and only work on that copy.
Jump to the end of the file. You should see a line that contains just "%%EOF" - potentially followed by a line end marker (e.g. ^M), depending on what editor you use. Now search backwards to the next line that contains this "end of file" marker "%%EOF". Delete everything from the line right after "%%EOF" to the end of the file and save as a new file with a new filename. You may just keep on going and search for the next to last "%%EOF" and save again, under a new filename. Then try to open the files you just created until you find one that is no longer corrupt.
This works because Acrobat, when you use "File>Save", will create an "incremental update" version, which is the original filename with all changes attached after the end of the original file. When you then remove this incremental update, you are basically restoring the version of the file before it was last saved. This will not work if you've used "File>Save As", because that will re-create the complete PDF structure form scratch, merging in all incremental updates performed up to that point.
If this is not working for your file, you can try to identify which page contains the problem by splitting the file down the middle: Open your document, then select the first half of the pages in the thumbnail view and select to "Extract pages". Then save the newly created file with half the pages under a new name. Do the same thing for the second half of the pages. At least one of these should be corrupt. Assuming there is only one half that reports the corruption, take that half and split it in two again and try to save (that would for example now be the first quarter of the pages and the second quarter). Do this until you have identified the corrupt page. You could then try to recreate just this one page and replace the original page with the newly created copy and fix your file that way.
Wow, how detailed instruction, many thanks dude! But I hope I would get something else