Just ran into this one, and would like to know if anyone has encountered this problem before. I had started a new game and had reached Riften for the first time. Rather than enter the city right away, I decided to talk to members of the nearby khajit caravan. By the time I moved toward the gate to enter the city, I noticed that a group of Imperial soldiers (clearly from a nearby spawn point) were fighting the guards. When the dust had settled, the guard who is supposed to demand a bribe was nowhere to be seen, and I was perma-locked out of Riften. Entering by the other gate wasn't an option, as clearly Beth wants you to run through their show-and-tell gauntlet from the main gate.

If there are any guards left, you can try and get yourself arrested. However, if the guard respawns, just wait out the respawn period.

There should have been a body, if the guard had been killed - so I don't think this one respawns. However, getting arrested unlocked the gates, so thanks!