Question: When stored for long periods of time (as in years) the Resin component may appear to have gone hard in the bottle. Is it now useless? 

Short Answer: Before throwing it out, try warming it up. If it returns to its usual thick liquid state, test a small sample by mixing Part A and Part B. If the bond strength turns out as expected, you should be good to go.

Better Answer: Try warming up the bottle of hard Resin. The simplest way is to stand the bottle in a large pot of hot water to a depth that covers all but the top of the bottle. Change the water occasionally as it cools down. You will see the resin gradually begin to change to its thick liquid state. When all of the resin is liquid, test a small sample by mixing Part A and Part B. If the bond strength turns out as expected, you should be good to go.

Another technique that works faster but that requires more care and attention is to ensure the bottle cap is tight and sealed. Then lay the bottle on its side on a concrete or metal surface. Using a hair dryer, blow hot air on the side of the bottle and roll it back and forth in the stream of hot air to ensure the heat is evenly applied. DO NOT FOCUS THE HOT AIR ON ONE SPOT or you may melt or deform the bottle. Roll the bottle back and forth constantly using the stream of hot air. When all of the resin is liquid, test a small sample by mixing Part A and Part B. If the bond strength turns out as expected, you should be good to go.

Note that when you mix epoxy that is warmer than room temperature, it will cure more quickly than specified.  

If you have any doubts about the strength of the bond always use Epoxy that is less than 1 year old. Fresh 30 minute cure Epoxy will give you the strongest bond.