I'm left wondering how this could even happen, how a man like Caesar could fail, but I'll try to answer your query. I'll assume that Caesar doesn't start a civil war, or else that his men are angry and disappointed enough not to follow him.
Caesar commits suicide; or reenters Rome to face immediate charges of incompetence, illegal warfare, and violation of a Treaty of Friendship; or goes into voluntary exile.
Instead of Caesar's victories bolstering him into the sunlight, his defeats mean that Pompey stays there as First Man in Rome. The conservative Senators (boni) continue to use him as a boogeyman, and example of what can happen when one man becomes too powerful. Pompey never marries Cornelia Metella; hell, the POD might even butterfly away Julia's death.
Being Caesar's son-in-law and erstwhile close friend, Pompey might well use his considerable influence to get Caesar acquitted of all charges; the alternative--divorcing Julia, whom he apparently loved dearly--is very unlikely.
So you have Caesar being acquitted through Pompey's influence, thus making a patrician Julius the client of a New Man's son. I think Caesar would prefer to go into exile with what little gold remains from the Gallic venture. From there--I really couldn't say what would happen.
The Republic is at an impasse. There's the boni--the conservative faction headed by Cato and Scipio Nasica--who can find new members easily, but don't have much influence with anybody not in the First Class of voters; and there's Pompey and his allies--plebeians and New Men, mostly. Since the only person at this time who was great enough to have a chance to eclipse Pompey was Caesar, the real storm would break when Pompey dies.
As for what happens then, I cannot say.