Yes, sure; this is a straight forward one (and easily fixed).

The GlassCast 10/50 resin is highly susceptible to crystallisation which in fact occurs at about 8 degrees C so what's happened is that the resin has got cold at some point between when you used it last and now. The good news is that the process is completely reversable and does no harm to the resin. It does however take a reasonable amount of energy (warmth) to reverse it.

To reverse the crystallisation, fill a sink or bucket with hot water (it can be as hot as you like really, the hotter the better). Ensure the lid is fully closed on the resin container and then immerse the resin container (containing the crystallised resin) into the hot water. Leave it there until the crystals, or any sign or cloudiness, have disappeared. This could take 30 mins or more so you might as well just leave it in there and forget about it for a bit.

Once done, the resin should be perfectly clear again and as good as new. It could even happen again in the future so try to keep the resin somewhere reasonably warm, especially as the cold whether approaches.