It depends how much tip you lost.
Tips are actually fairly easy to grind -- you should nearly always work from the spine (top) of the knife and leave the edge alone. Just blend the top into the edge at whatever angle you like. You don't need to keep a curve or to match the manufacturer's angles.
Obviously, it's easier to do with an electric grinder than a stone. Naturally I always work with stones. After you've ground a new tip, don't forget to polish it as much as you're able. Sandpaper is probably best if you're using hand tools, and buffing wheels if you're using a power grinder.
If you've lost a lot of tip, you'll definitely want to hunt down someone with a grinder -- ideally, someone who is also competent to make the repair for you. If you've only lost a little, you can easily handle it yourself.
Files fall somewhere in between stones and grinders. Just incorporate the advice for stones.
One thing though -- if you're using a stone or file, stroke from front to back (and vice versa) along the knife and not from left to right across it. Obviously, that doesn't apply to sand paper -- that you'll want to "shoeshine" across the knife. Not only will it take the rough off, It will "crown" the edges of the spine -- a good thing in itself.
Hope this helps,
BDL