The previous mud be probably crumbled for one or both of these reasons: the sub floor flexes and the bed was not reinforced with metal mesh and/or the bed was constructed from inferior concrete mix.
Two possible ways out...
1. fir up with wood and plywood and put concrete backer board over it, then tile 2. Make a new mudbed out of thinset mortar and reinforce it with metal mesh.
I would probably do #2. Use screws to secure underlayment as firmly as possible to joists. Lay down some heavy galvanized mesh, maybe 1" squares (doesn't really matter) and/or some pieces of steel rebar (reinforcing bar like they use in foundations, etc.). Rebar should be one piece every foot min. (approx.) in a checkerboard pattern, i.e. running both ways. The mesh and/or rebar will provide tensile strength to your mudjob and help prevent cracking from floor flexing. Buy and mix up "thinset mortar" to create your bed (the same stuff you will use to set your tile). Either buy the stuff with the latex additive already in it or purchase separately as latex bonding agent called "milk" in the trades (gallon jugs of white liquid). This can be added instead of and/or in addition to any water. Mix the mortar for your bed as dry as possible, barely enough liquid for it to hold itself together and not still be powder. This should not even be close to being soupy and not "slump" at all. You need to make sure that a little bit of this bed mortar lies below your mesh or rebar. It may be easiest to just trowel some in your area first, maybe 1/2" deep, THEN place in your mesh or rebar, THEN trowel more in on top of it. Figure out how high to build your bed, i.e. if your tile is 1/2" thick and your thinset mortar below it will end up 1/4" thick then you need to build your bed up to 3/4" of where you want your finish floor to end up, so if you have a 2" drop maybe you want a bed 1 1/4" thick. You might take a straight piece of 1x board and take a 3/4" notch out of one end corner, hang it over your finished floor to use as a screed, i.e. to scrape your bed to finished level relative to surrounding floor and to make it nice and flat. Let bed dry for a day and then install tile, let tile set for a day with no walking on it, then grout the next day.