In order of probability:
old battery gave up the ghost
corroded battery connection at terminals
corroded battery black ground wire where it connects to the frame
if you have a volt meter you can check the battery on its lead posts while you crank it.
if it holds up to 11 to 12V, then check it again on the terminals on the wires. If you lose the 12V there the terminals are corroded
If you still have 11 to 12V there, put the black lead on the frame somewhere clean, like a bolt on the rear fender rail.
if you lose the 12V now while cranking, its the black wire where it connects to the frame (will have to trace the wire to find the connection).
If you get thru all that and are still seeing a solid 11 to 12 V then you will have to check the starter relay.
Last edited by KCW; 04-02-2019 at 03:22 PM.