It is rather difficult to guess what cap belongs there and what else is missing. Form the position close to the power transistors, this could be a afterthought (and thus on the back) cap to reduce oscillations. These caps can be tricky to guess, even if you have a circuit diagram. The capacitance can matter and could be anywhere from 10 pF to about 10 nF, depending on the circuit.
Without the schematics its really hard to tell what is missing, and even than one might need to do a simulation or/and measurements to see, what is missing / wrong. If you are lucky the cap is just extra buffering and less critical, at least that could be seen from the schematics.
The resistor (100 Ohms ?) that is still there at the under side looks damaged too. At least this one is easy.