the car battery may not have enough voltage , bmw oem battery has small round window on top , if you look straight down you will see colour , if green then battery fully charged . if black then needs charge . if yellow then new battery required . any battery voltage can be checked with voltmeter . your car has obd2 diagnostic that can be read with a diagnosis tool that can identify many problems within cars electronic and electrical system and should indicate what the problem is . my 2002 4.4i has done same problems after driving in rain although usually starts on second attempt . i have read other threads on X5 world that some cars stop working after driving on very wet roads then start after a few attempts. maybe yours drained battery through water in connector someplace. it is possible to do some diagnostics on dash display by pressing buttons for trip counter and check control (the two buttons between speedo and rpm ) you will need to search this thread , try searching obd or diagnostics . if you dont find anything here try www.e38.org i think instructions may be there somewhere. my starter was required many attemts when i had coolant leaking onto it , stopped leak then car ok till on wet roads so could be corroded connection vulnerable to water. GOOD LUCK WITH THAT.