Thanks for the quick response ,

I never backed up my files before i did the upgrade ( how stupid) I tried to upload the upgrade package of DNN 4.9.1 and removed the entries in the DB for 5.0 and tried the upgrade again. 

Sites totally unresponsive im getting

Server Error in '/' Application.

---

Runtime Error

Which is not helping the situation at all.

What i am thinking , the only way I can see me restoring my production web site is , to reinstall 4.9.1 clean, back the remains of the current corrupted DB, and then transfer the data from the key 3rd party module tables, Users tables etc to the new clean version.

It is a long process but it should work,

Anybody any advice on this move, i see it as the only way currently.

I am amazed that DNN has not got a roll back function, or a away to remove the 5.0 files and DB entries and replace with the 4.9.1 files and entries.

If this method is the only option open to me its going to be a long long night.

I just hope my hosting company tells me they can restore the database.  That will be half the battle , then i would just need to make sure i have the 4.9.1 files uploaded right .

My hosting company was able to restore the database to yesterday and they also said they were able to restore a back up of the files also.  I am so relieved, a big lesson learned today, BACK UP , BACK UP, BACK UP i could very easily have lost about 10 months work there.   

Good news is my site is back up and looking good, there is a little issue though I cant login,  when  i click login i get this message apperaing

A critical error has occurred.
Method not found: 'System.Collections.Generic.Dictionary`2<System.String,System.String> DotNetNuke.Entities.Portals.PortalController.GetPortalSettingsDictionary(Int32)'.

Is there any one in the community who recognizes this and is able to advise me. 

kind regards thanks to everyone who replied already I appreciate the community feedback, advice and assistance.

I was able to rest the style by replacing the skin folder,

Im still trying to resolve the issue i get when I click on login. 

I tried updating the DLL's in the bin folder but still same issue.