Went to mix a bit of epoxy today to fill a minor ding and found that the resin part had gone hard in the plastic container 
I think its about 3 years old but I just assumed that it would have an indefinite shelf life if not mixed. Maybe some brands are better than others for shelf life.
depends on the product- many will have a best before date.
If the product is opened however it may only be good for 1-2 months.
It will absorb water from the air and it goes smelly. But epoxy is a two part reaction. In other words the molecules in part B bind with the molecules in part A.. This is why you should never change the ratios.mmaybe you put the wrong spoon in the wrong pot?
If it's boatcote just put it in the sun for a few hours. It's like honey, in cold weather it crystallises.
If there's no sun a micro wave on low will also do the job, but make sure there's no leaks other wise there could be domestic issues!!!!
I've used boatcote for many years, and this time of year, it's common for me to wrap the resin in black plastic and leave in the sun.
If it's not botecoat, try my fix anyway, it might just work.
If it doesn't liquefy after several hours, it may have been contaminated with hardener, if that's the case there's nothing to do but throw it out and buy some more.
Most likely you used the same spatula for mixing and insert back to main container.
I have some bottles more then 10 years old and still strong...
Decrepit was on the money, put the container in a bucket of hot water from the tap and came back in an hour and it was completely liquid again!
Fantastic, glad that worked.
I've found as the resin gets older it takes longer to get it liquid. With my very old resin, I don't think an hour in hot water would be enough, so allow a little bit longer next time, you may have to refresh the hot water.
Just a small tip...
sometimes I just prepared too much mixed epoxy and I have left over...
pity just to throw away .. so I put to a freezer to slow down solidification ... then
warmed up after a month .... and sill works fine ...
