Is it possible to restore teak furniture that has been stained dark to original finish?
Teak is one of my most favorite woods. It naturally has a high oil content that resists much penetration. While it can fade to a grey-ish look, light sanding can restore it easily. 

In your case, a mild citrus stripper such as CitriStrip is environmentally  friendly and pretty effective.  Just experiment with applying it to parts, let it be for a while (like 30-60 minutes) and wash it off.  Learn as you go. A wire brush or steel wool are also good tools, but if you use steel wool, be sure to completely rinse and dry the area, steel wool can stain wood. 

Let it dry for a while and then oil it.   It should be good as new.