BMW Repair: starting problems, bmwx5, catalytic converters

Question
While starting my BMWX5 in the morning,it started and ran for a short while and then quit and when I tried to start it it would not turn over it just turns the ignition on and will not engage the starter, I since then replaced the battery thinking that this was the problem, and it does the same thing , my BMW is a 2001 and has about 85,000 miles it has just received 2 new catalytic converters and new water pump and the belts have been replaced, my question is could this be a safety item that is keeping it from turning over or is it more like a fuel pump but wold that disable the starter,I am quite perplexes at this problem can you help me please

Answer
Replacing the battery may have been good as the battery might have been original.The problem maybe the starter though.You first need to check to see if you have power to the starter,then check to see if there is power when turning the key to the start position on the wire(smaller)on the solenoid. Checking the grounds to make sure they are clean and tight as well