Hi everyone need some help please

Hi just wondered if anyone else has had the problem that I've had today? So when I turned off the key on my 90 200 tdi I could hear a noise like a fan! I realised it was the starter motor spinning and the key was in my hand !!!!! So after a quick panic I tried to take off battery terminal but was not quick enough! And it burnt out so have fiddled it off eventually and ordered a new starter. Do we think that this is the problem as for the last week or so it's struggled to turn engine over even with fully charged battery? Or could it be the relay or ignition? Any help would be much appreciated . Thanks