I need to replace this capacitor but don't know exactly its model and where to find it.
Can someone help me with this?
It is 100nF (100000pF = 104). +/-20% tolerance
It needs to be rated for adequate voltage for your circuit.
It’s probably a junky Z5U or similar, but you can replace it with a better part such an X7R dielectric capacitor.
If this was a decoupling capacitor, then the circuit should work fine with it missing. So if the circuit doesn't work at all, then something else is likely broken in addition to this capacitor. Unless you were very lucky.
If the circuit works and you're not well experienced in PCB rework techniques, then I would just leave it be. You may do more damage by replacing it than by not replacing it.
The capacitor can be any 100nF axial ceramic capacitor with similar size. Use the search function on Mouser, DigiKey, Allied Electronics, Newark, ELFA/Distrelec to find some.