If you have an OS installed is simple, just use hponcfg and you can change the password as explained in https://blog.toma.guru/2015/04/hp-ilo-linux-reset-password.html but if no OS is available, then hope is not lost, you can use the iLO Physical Presence Button.
On RX2800 Itanium iLO Physical Presence Button is hidden behind the small red hole
As stated on https://support.hpe.com/hpesc/public/docDisplay?docId=c02728748
The iLO 3 physical presence button enables to reset iLO 3 and reset the user-specific values to factory default values. A momentary press causes a soft reset of iLO 3 when the button is released. The iLO 3 Physical Presence button enables to reset iLO, enter TPM physical presence mode, and enter security override mode.
A momentary press of the button resets iLO and clears any security override or TPM physical presence mode that were initiated by this button.
A greater than 4 seconds less than 8 seconds, press of the button places the system in physical presence mode for up to 15 minutes.
A greater than 8 seconds less than 12 seconds, press of this button places iLO into security override mode for up to 15 minutes. Security override mode enables to enter iLO without being challenged for a password enabling to set up users.The UID LED blinks once after holding the button for 4 seconds and once after holding the button for 8 seconds to help gauge how long the button press has been held.