As for the notes, make sure you dont have anything playing on ANY of the tracks because sometimes all it takes is a scroll down to realize you have a track there playing music.
And for the tempo. You can adjust each track by double clicking on in, then turning the power on in the "tempo and tuning" section and adjusting it there.
You must have reduced the length of a object too much so that it disappeared, but still will play. You have to find that spot by clicking on in and around that area until you see the scissors symbol light up, then you know there is an object even so you don't see nothing, and then you click on the scissor symbol and eliminate the invisible object.
It may be on another track as a previous answer says, otherwise
I assume you're talking about midi objects.
I agree the note's probably been shortened until it's nearly invisible.
Either stretch out the object until it becomes visible, or click on the events list then click on each note until you find the offending one.