Radiation effects from two-level atoms caused by the presence of a Fock-state field in a damped single-mode cavity are examined. An exact recurrence differential equation for density-matrix elements valid for any number of atoms or photons is derived. Numerical solutions for different cavity dampings (low- and high-Q cavities) and different photon numbers of the Fock-state field being present initially in the cavity are presented. Collective effects caused by the presence of many atoms are investigated. It is shown that new quasistationary states as well as collective radiation inhibition appear in the time behavior of the atomic energy expectation value. These effects arise from averaging over an ensemble of which each member consists of N identical two-level atoms placed into a resonant single-mode cavity. A comparison of the obtained results with those for an initial coherent or thermal field is also given.