<<< All of the sudden, within a day about 2 days ago, a couple fish died. i noticed a couple of dead crabs also. Also some other things are on the verge of dying >>>
Most crabs are generally very hardy. You either have something killing them along with your fish, a disease of some sort, or a serious water quality issue with one or more parameters way out of whack. If various corals are not opening now, that leans towards a water quality issue.......check everything, including if something went bad in the tank like a powerhead or whatever.