If it's a stuck starter relay you can try tapping/whacking the relay with a tool and maybe it will un-stick. Once they stick, they are more prone to stick in the future, even with a good battery.
You can replace the relay (they're not cheap) and be careful to use a good battery with good electrical connections.
I have disassembled relays and sanded the previously stuck connections so they'll be less likely to stick. One is in my bike now.