The silicone is probably dried up & so you have a leak. It is fairly easy to reseal a tank. If you can do caulking around a bathtub, you can do a tank reseal.
Instructions here:
http://aquariumadvice.com/forums/f20/resealing-tank-s-8320.html
My additional tips:
1. clean out all the old silicone on the inside of the tank, redo all the seams. <If one is old & leaking, the others are going to be going soon, so may as well do all of them at once.> You do not have to take apart the outer seams (ie the part that is holding the tank together) unless there is a structural problem .... like loose pane of glass.
2. A sharp razor blade & skew chisel is what I use to remove the old seal.
3. Clean the area of all traces of old silicone, then wash with vinegar.
4. Use masking tape to lay out the seams so you have nice straight seams.
5. Apply a nice bead of silicone between the masking tapes, smooth with gloved finger (non-textured neoprene is best), remove masking tape while the silicone is still wet.
6. Wait at least 48 hrs - longer if your silicone instructions say so - before test filling.
