There would have probably been no President Reagan.
You don't think Reagan would have made a second play for the Presidency in 1980? He was considered a front runner as the next GOP nominee; He had just barely lost to Ford for the nomination. Ford would not have been able to run in 1980 and thus the GOP primaries would've been wide open.
Wait, how? No one can be elected POTUS more than twice. 1976 would have been the first time he was elected. Now, given his probable unpopularity in 1980 due to the Iranian hostage crisis, he might choose not to run again, in which case Vice-President Dole has to duke it out with Reagan for the primaries. Dole would probably end up with most of the major dark horse candidates (Bush Sr., John Anderson, etc.) behind him. Reagan might still be able to pull it off, though. Now, if Ford did decide to run again, Reagan would have a very good shot at it. Though whoever the nominee would be would probably have a difficult time overcoming 12 years of incumbency fatigue, which included both Watergate and the Iranian hostage crisis. So it could allow someone like Lloyd Bensten, Mo Udall, Frank Church, Birch Bayh, Jerry Brown, Walter Mondale, or Ted Kennedy to win comfortably.