Constructed own pc from old parts- issue with booting up

Dear All,
I have managed to set up my own gaming-rig with parts obtained from different PCs. I have checked all the individual components and all of them are in working condition. When I finished installing all the components and switched on the system, it is shutting down with a constant beeping sound. When I managed to switch it on the first time, I was told no OS was detected.
