Is the bottom end of the tube still soft? If so, puncture that end of the tube to see if it works. If it's silicone-based sealer, it undoubtedly is completely solidified. I've never had the stuff, once open, last more than 2 years. I do have a tiny tube of Seam Grip (not silicone-based) a little over 2 years old which is completely plugged, but the bottom end is still soft. I haven't tried it to see if the sealer is still usable, though.
IMHO, 5 years is an awfully long time to expect most substances to last once they have been opened.