From the horse's mouth:
http://www.ilfordphoto.com/Webfiles/2006130218312091.pdf
Fixer has a limited shelf life, even new in the bottle. If your fixer concentrate smells of sulfur and/or you see copious yellowish/white precipitate, then the fixer has likely "sulfured out" and is no longer good. Read up on the shelf life as well as the capacity and times in the document linked to above. You can use a piece of film for a clip-test to test the fixer's activity.
Refixing your prints in fresh fixer may save them, especially if they were stored in subdued light. Often, however, there is a bit of light-reduced silver fog that remains. You may be able to combat this with a weak bleach (I've saved some prints from old, fogged paper that way). Soak - bleach - rinse - refix - wash. Do this with all the lights on so you can see the effect. Be careful not to over-do it; you just want to clear the whites. Practice on your worst prints first.
FWIW, I think saving used fixer for longer periods of time is bad practice. I mix what I need for a batch, or a session. When doing lots of printing, I'll keep my fixer for a couple of days, discarding bath one and replacing it with bath two as throughput capacity is reached. However I never keep it even close to the published storage life. For film, I try to mix just what I need for a session.
And, as mentioned above, don't use fixer already used for film for prints; the by-products don't get along.
Best,
Doremus