Most important to say what model Optiplex and OS. You do not need to see the Dell logo to get to any pre boot utilities. It might go by too fast to see. Just start tapping F12 immediately when you turn on the computer without waiting for any screen. The boot menu will let you boot to a diff drive so you don't need to get to bios. 

Tapping F2 also works for the bios.

Thats not the problem.

Mine is 5040 optiplex windows 10.

I can enter the bios do whatever i want the only thing is why i can't see the dell logo. On BOOT!!!!????

That is fast, and fast for an HDD.  Your 5040 must be pretty nice.

You're welcome.

@bradthetechnut​ mine dell logo also not showing and computer also not booting up (model - dell inspiron desktop [all in one]) i have hdd in my computer and just installed windows 10 . Only black screen can be seen .

@SHOTGUN-3​ 

You may want to start a new thread in the Inspiron forum, as this one is for Optiplex's.  It needs to be said what model # your Inspiron is.  I'll still give an answer here with some ideas.

Does another boot drive work in the Inspiron?

If that Inspiron is 5+ years old, replace the size 2032 CMOS battery.

If still no luck:  Does it at least let you access F2 BIOS?  If not, do a BIOS reset.  Use jumper method if your MB has it.  If your system is newer, you may only have to push and hold the power button for 30 seconds.

I'm going to take it the Inspiron in regards has UEFI BIOS.  Is it set to Windows Boot Manager for Boot Sequence?  That can also be checked in F12 Boot and Diagnostics menu.  Might not hurt to run Diagnostics and see what it says.

Make sure BIOS is set to UEFI and not Legacy (if it has the Legacy option).  AHCI and not RAID.  Secure Boot on.  Reload Win10.  When the Window pops up asking what partition to load Win10, delete all previous partitions.  From my experience, there might be 1 or 2 partitions, like System for one, that don't delete because the system needs those.  Then click on Install, and the necessary partitions will be created and install continues.

I'd also like to suggest using an SSD.  Just about everything goes faster with it, like boot times, Windows Updates, restarts, program loads, and etc.  Win10 restarts are painfully slow on an HDD for some reason.