If you do everything that Barry has posted and still have the problem, you may have the same issue I had with my '81 R65 about 30 years ago .
The starter relay will make a faint click and nothing else happens when you press the start button .
Remove the fuel tank and press on the top of the starter relay, you should be able to manually close the contacts in the relay and the starter will work, just make sure the transmission is in neutral, because you bypass the safety interlocks on the starter circuit by doing this .
You can remove the plastic case from the relay and adjust the movable part of the relay a bit, doesn't take much to get it adjusted .
You'll need to remove the relay from it's socket to do this .
You'll probably need to use a screwdriver to get the relay removed, use caution, there is a ' hot ' wire on the connector, it has power from the battery all the time, regardless if the key is off .