Well you should of reported it really but i can understand why you didnt , i think the best thing to do is contact the police a,s,a,p as there may still be some evidence of the theft
the insurance company will try not to pay out by claiming you havent reported it to them in time , but as you probably already have explained the situation to them they may fight your corner if you can provide some evidence
beaware it is also an offence not to report a crime so you may be liable for arrest
the easy option is admit liability for the crash claiming you rang your insurance company and asked the