When upgrading processors, a BIOS firmware update will certainly helped.  But depending on the shipped date, BIOS firmware with supported microcode may already applied.  You can try to reset CMOS via jumper to see if new processor can be recognized.  Note that there is no overclocking option for K processor.

May need to reinstall previous CPU and update BIOS.  Then reinstall i7-7700k.

Make sure no pins are bent.  Also make sure old thermal paste didn't inadvertently get between CPU and socket.  It happened to me and I luckily had the impulse to double-check things before turning on.

Thank you for your help.

I have replaced the original cpu, monitor worked ok, reset CMOS now I have no video out put from my graphics card, on board HDMI or DP.

When it re booted, it seemed to go through a diagnostics set up but I can't see it.

I'm leaving it on for a few hours to see if it will turn the video back on after it's diagnostics. 

This is going from bad to worse

Try without the gpu.

Also, is the ram normal or ecc ? because if ecc, it will not let a normal cpu boot while the xeon will boot

Removed gpu, memory but nothing. Removed the button cel and re set cmos, nothing - then a bit later sprang into life. Had to do battle with the bios set up all over again. Forgot about raid setting, and legacy / uefi, all sata cables in the wrong holes   but it's back.

Memory not ecc, bios=1.2.4 anyone know if this old bios is ok with a i7-7700k ?

Thank you for your help.

probably the bios is a bit old , they moved to version 2.x since 2017

Latest BIOS for the 3620 is 2.25.0.  Dell recommends not jumping too many BIOS versions at once, but do about every other until caught up.  The part I don't like about that though is 2.10.2 gave speedstep (former Rockstar on this forum) trouble.  I can't remember if he was able to do BIOS recovery, but I think he got it going again.

I just found a Dell article.  Skip 2.10.2 if it lets you.  Precision Towers 3420 & 3620: System hanging at the Dell Splash Screen or failing to POST 

All bios up dated, 2.10.2 seems to be no longer available.

Cpu changed for i7-7700k

One of those tense moments when the monitor doesn't light up, then the fans stop, and start again, then the little green led on the monitor puts a smile on my face. All done.

One question before I go, the cpu is 4.2ghz, it take it that it can achieve it's turbo speed of 4.5ghz?

Thank you for your help.

Yes, the turbo boost will kick in when the condition required.  You can test it by running a task like Windows Security full scan to trigger full CPU load.  At the same time, opening Task Manager and monitoring CPU in Performance tab.

Glad that you have a successful upgrade.