I have a xj 750 seca and I'm not getting spark all of a sodden. the bike was running a week ago and i was waiting on a new exhaust. I put the exhaust on last night and the bike wouldn't fire up. if any one could be of help thank you

Let's start with the basics.
When you said it wouldn't start, did you mean that you could turn the key and depress the start switch and it turned over? Or that nothing happened?
Have you checked the kill switch? Had that little thing bite me (and countless others it would seem) in the can a few times.
If you replaced the exhaust, did you perhaps disconnect the neutral sender?
It is located down there and it will disable your safety interlock system.

thanks for your reply it was a relay switch that was stuck. once again thanks for your help.