Often happens. In fact it is pretty much inevitable with any fertiliser mix that contain salts that are hygroscopic <"Hygroscopic compounds – Hygroscopic Cycle">.
You <"can still use it">. The easiest option is to oven dry the mix at a fairly low heat and then smash the mix back up with a rolling bin. If you want to keep it dry after that, either in an air tight container or the freezer.
And you can reactivate little bags of "silica gel" in an oven set at about 120C for an hour or microwave for a few mins. Lots of online guides for this. Careful of the temp because you can destroy it if it is too hot, so check the manufacturer instructions (If you can find the name on the pack).
You can, and you can also tell whether they've absorbed water (even though not when they are in the sachets), yellow is hydrated and purple dehydrated.
