"You will have to replace the fan".

Thank you - how is this done please?

Booting and Fan Failure are not related.

You will have to replace the fan.

this step is in your  service manual, and free to read 24/7 at dell.com

you buy a new one. (hint ebay has many) the wire count matters, not just any fan only dell fans.

then unplug old, connector

then unscrew the fan.

then screw back new fan.

then put the connector back

power cord pulled , ok>

really RTM read the manual first then ask better questions, is always best.

black screens

there are 12 version of BIOS here,  A5 -A23 and more below, I'd not pretend to tell you what all version do

if the FAN RPM is 0, not me, ever. ( Dell does not DOCUMENT BIOS at all (or half way) or for sure the deep ever)

but id OPEN the case and see if the fan connector fell off or OMG so common, packed solid in lint, even 1 year later. and stalled in LINT,  done 1000s this way, packed. (PC with zero service)

why not look first, when BIOS tells you fan is dead,  look first. open case.

and black screens with words (errors in white text) really is not a black screen  it's black and white screen.

totally different than dead screen black (but I know what you meant) I thinks.....

The size of the unit dictates how the fan is replaced.

They are not all the same for MT DT SFF USFF.

http://downloads.dell.com/published/pages/optiplex-990.html

4 Sizes for Models this old

Booting and Fan Failure are not related

until the PC overheats and shuts down overheated, so is related, but yes there are 2 problems, as is the case buying old used  computers.

no CASE SIZE STATED, BUT THE OP NEVER ASKED FOR A FAN PART NUMBER.?

here are the four 990s  pickone.?

No such thing as one size fits all.