Thanks for the hints, I had previously tried most of the start up options, but was unable to cure the problem of it constantly shutting down after entering my password for the user profile giving me the problem. I found a way into my downloads and files using another one of my login profiles, which I didn’t think would be allowed with Catalina security.
I then backed up the PC using Time Machine usb drive. I then tried loading a previous backup from my Time Machine which took an age only to inform me at the end it couldn’t retrieve some of the files. On boot up I had the same problem with it continuing to shut down, only this time after only a few seconds.
I went into recovery mode and downloaded Catalina from Apple. The Laptop started up and I was able to access my troublesome profile, and on inspection I found the backup I had previously tried from Time Machine had actually replaced the troublesome files.
So I’m up and running again and have made another backup. I'm sure the original problem stemmed from me transferring downloaded files into a KeyChain secure folder, and as yet I have not attempted to enter KeyChain again.
But, the BIG question is, will the Laptop start up again when I try the first Restart … fingers crossed. It has been a few frustrating days and nights …
I was ready to completely wipe, reformat and start with a clean startup.