Bios default password?

Dell OptiPlex GX520 PC Desktop:

What is the default password to get into the BIOS, to set up the boot sequence.

The BIOS default password is just blank, and it is not asked.
When you set a bios password, you are asked to enter a value, therefore if bios is asking the password, the password had been changed.
To reset the bios password if you do not know the password you need to access to motherboard and temporarily remove battery for few seconds.
Removing battery will reset bios and delete the password.
Unplug power cord before accessing the motherboard, and try not touching the motherboard with your finger.
Put battery back in place before restarting computer.

Can't get into the bios?

Dell OptiPlex GX520 PC Desktop:

It may be a damaged bios ..and in this case you need to reinstall bios ..download the firmware and the instructions from the support site ..
It also can be a different board inside..and will not react to normal F2 ..in this case try to hit dell on start ..If the board was changed ..this may be a case..Or f the BIOS settings were updated this can be also a case..
try also F10 on start ..just in case it has a hp board inside..
let me know what you've tried already ..and what exactly it does..and if you are able to start it normal ..
If you managed to get into bios ..try to default the settings to the factory values..This will make it back like It was..
What is worrying me is the fact that you do not have the dell screen .It is either disabled or it does not boot at all ..and in this case it is something else to blame ..try what I said just in case it displays teh devices and starts in a way ..and if it does..try to press teh F2 repeatedly ..just in case you do not manage to get the timing right.