Someone on the forum here (Datchew?), jokingly responded to every electrical problem with "it's the battery", after replacing everything else first.
The battery is the FIRST place to inspect - and make ABSOLUTELY sure it's tops! Only then move on. Even though it reads 12.5 V it may not have enough capacity to start the bike. It needs to be load tested - an in-home test is to see what voltage it drops to while cranking. Anything less than 10 or 9.5 and it's got problems.
Of course, there are other things that can cause a starter to not work, but this is the place to start.
Oh, and be very precise in describing the symptoms - it'll help enormously.
Wanted: Dead, smashed, crashed or trashed gauges
BMW GAUGE REPAIRS - TACH*SPEEDO*CLOCK*VOLT METER *PODs & LIGHT BOARD*