What if we say no to God? What will He say?
Well - we say no to God all the time - what did He say? Every time we are tempted to do something wrong or let wrong thoughts inhabit our mind or do any sin at all, the Holy Spirit warns us not to do it. The more we pay attention, the easier it is for us to hear the warning. The more we ignore the Holy Spirit, the harder it is to hear the warning. Eventually our hearts can get so hard that we don't hear God's warnings any more. So we all have said no to God at some point in time.
If it is about something that God is asking us to do and we say no, then I guess it depends on how badly God wants us to do that particular thing. The story of Jonah is a case in point. Jonah not only said no to God when God told Him to go to Ninevah (Jonah 1:1-2) and preach, but he sailed off in the opposite direction (Jonah 1:3).
Eventually, a storm from God threatened the ship (Jonah 1:4) and the sailors discovered Jonah was the problem (Jonah 1:7) and eventually threw him overboard (Jonah 1:14-16) at which point the storm stopped. He died, and was swallowed by a fish who spit him up on the ground three days later (Jonah 2:2-10). After his resurrection, he went and preached to Ninevah (Jonah 3:3-4) and the king and the people were converted and God didn't destroy them (Jonah 3:10).
When it is something that God really wants done, then He can get our attention and make us want to do what He's asked. It's always better to just do what He asks in the first place if we are sure it is His will and lines up with the Bible in all respects. God is always pleased with a heart that is soft toward Him.