Immobilised Dreadnoughts can still fight back in an assault, but their number of attacks is reduced by 1 for each immobilised result they have received. Krak grenades and melta bombs are useless against a mobile Dreadnought but they can be used against Dreadnoughts if they are already stunned or immobilised at the start of the close combat.
Immobilised Dreadnoughts In Close Combat
Source: Core Rulebook 3E, p. 91
An immobilised Dreadnought can still fight, but loses one Attack for each immobilised result it has suffered, and becomes vulnerable to krak grenades and melta bombs.