How long have they been in there if you just found them?
I have found that forgetten crab legs tend to have the meat shrink up and they aren't even worth it. I'd snap one open to take a look before putting any effort into them.
That said, just warm them through. Sometimes I put them in a pan with water to steam, sometimes I ad Old Bay or creole seasoning/sometimes not. If they are thawed they reheat even better. Just wrap them in a damp paper towel and nuke them a little bit. I saw that one on AB and it works quite well.
Like I said, the trick is to warm them through. They are already cooked.
Thaw them in the fridge, then check them as Pac suggested. If they're okay to use, I would steam them and then dip them in melted salted butter and eat. They don't need much preparation since they have such a delicate flavor.
I just played around using pretty much the ingredients you mention, JG. It is a great accompaniment to seafood. I usually keep the crab legs cold and just dip in the hot butter and chili lime mayo. Enjoy your unexpected treat!